Is there any way to train assessment or scouting outside of books and one time practice sessions?

I keep using assess in combat but it never increases the skill.

Assessment won't increase with use so your answer is no.

Scouting can be increased to max.
When travelling move slowly, which can trigger a scouting increase if an unseen ambush is detected.
Once detected, move slowly back and forth within the immediate area to trigger more scouting increases until attacked.
Gorath has the highest Scouting skill initially, so equip him with a scouting ring to increase it so you can start triggering scouting detection more often.
The ring can be purchased at a jewelry vendor or looted later in the game.
All your characters should be wearing elven stealth boots which will allow them to gain increases for scouting and stealth when moving slowly.

I was just about to ask about stealth.

I just started the game, and I'm the kind that wants to max out the skills with these kind of games. Is it possible to milk one ambush for multiple skill increases? My attempts so far haven't been promising. I'll try to get a hold of those boots when I can afford them.

Yes! Milk it as much as u want.
I no longer bother increasing scouting. But when I did, as I approached an ambush, if someone get's a scouting award, I would save the game. Then u take about 5 steps back, and 5 steps forward. If the scouting award happens again, save the game and redo the same thing. if it doesn't happen, reload, take 5 steps back, and 5 steps forward again.
So once u find that sweet spot, keep working it.
I use assessment in combat from time to time, but as it, and scouting, don't contribute much to combat, I no longer put any effort into increasing those skills.
By the way, it might be 5 or 7 steps, so just save it, and play with it a little bit to see how many steps are required.

I believe scouting does give you first move in combat if you detect an ambush which is important if you go up against multiple spellcasters.
Do save often since there is no autosave, because if your group is frozen and dies, the game is over.
Krondor is unforgiving that way, but otherwise a great game.,

Agreed. That's why I said assessment and scouting don't contribute MUCH to combat. There are a few battles where having the first move is a big plus, especially if your characters don't have decent gear and little training. Chapter 5 battles, Nighthawks, Nago, etc. But I now get the choice weapons, armor, spells, combat items, and money early in the game by exploiting the weaponcraft skill at Yabon, so I can focus on developing melee, defense, and crossbow skills.
So I'm not saying these 2 skills don't have any use, but by saving before each fight, many times the enemy does something stupid and gives u the upper hand. The Nago fight is a good example. Owyn can freeze Nago with spells, if he's given a chance to cast a spell. So u reload the fight till he's able to cast Grief of a 1000 nites.
Another over-rated skill is barding. Owyn is the only one worth training, if u really want a 100% barding skill, just buy lutes and give them to Owyn till he reaches 100%. Taverns aren't necessay at all.
Best way to make money early is to buy, repair, and sell 8% Goblin Sticker swords.
Best money maker is the broken elf crossbow.
Best tip for increasing melee skill is to use good armor and a horrible wrapon, combined with bad health.
You want Owyn to freeze an enemy so Locklear or Gorath can use a 'thrust' attack to generate a low damage hit. The more hits this enemy takes before dying increases ur melee skill. That's why u use a very poor weapon (10% broadswd?), and want their health low (strength decreases as health decreases). That's also why u save the game before each fight.
By the way, I use assessment to determine when the last enemy can only take 1 more hit before dying, so I can get my characters healthy again before finishing off the last enemy.
That's why it so important to grab that Grief spell early in the game. It allows u to develop ur characters faster.

In Chapter 5, combats 24, 25, and 26 each have 5 enemy. 24 and 26 ONLY have 2 spellcasters, while 25 has 3 spellcastors. Spellcasters seem to have low stamina and health. In this case, the data shows 53 points or less.
The Elven Crossbow provides 25 damage points, and an Enchanted Quarrel provides 30 damage points, for a total of 55. I haven't tried it, but it appears that Locklear and James, if trained and equipped, could each do a one-shot kill on the magic casters. Patrus is the worst spellcaster in the game, but he has the Evil Seek spell which can hit multiple enemies.
There are 3 chests that can be accessed before any combats, which should have been filled with weapons, armor, spells, and combat items by u before the end of Chapter 3.
So the beginning of chapter 5 is tough, but u can save before each fight, so you'll win eventually, if ur guys are equipped and trained.

I tried increasing the assessment skill using combat, but it didn't increase.
I mixed up scouting and stealth. I believe scouting can be increased, but I'm not sure about strealth.
I got tired of wasting my time trying to increase these skills, and I never have found any clear explanation of each skill, their purpose, and how to increase them, aside from various classes, lectures, and occasional trainers. Being able to have the initial attack would be great, but for me, it's just like the Mind Melt spell. It's something that should work, but the developer dropped the ball.
I get around this flaw by using the 'save game' feature, providing money, the best weapons, armor, spells, combat training, and especially combat items. Good luck.
If I missed anything regarding getting the 1st attack ability, please respond, but i wasn't able to figure it out.

Usually I like to max all the skills. There seems to be no way to disable stealth, however, so if I max it I'll be permanently barred from all ambushes, and possibly traps as well. I was going to train it, but now I want to avoid that skill.

Thank you, you guys.

Yeah the problem with maxing stealth is as you say, it becomes very difficult to trigger ambushes, and then you have less opportunites to increase melee skills.

I would advise NOT increasing your stealth skill at ALL. This is because the stealth skill can be used to gain first strike on enemies you can see (good thing!), but it also causes you to avoid ambushes (believe it or not, bad thing). Any combat you avoid is a missed opportunity to increase your combat skills. If you increase stealth too high, you literally cannot be ambushed (even if you trek back and forth over the trigger like a duck in a shooting gallery).

That being said, if you don't care about missing some combats, the best way to actually increase your stealth skill is using Dragon's Breath & weed walkers. In the same way mentioned above that you can increase scouting, if a party member mentions you are about to be ambushed, bookmark the game. After you bookmark, cast Dragon's Breath & bookmark again. Take a step forward, if you enter combat, reload. If you don't you will have gained stealth xp, even if the skill doesn't rise. Take a step back & bookmark. Repeat as desired.

The best use of this is to slip past the Shades to get into Sethanon for Grief of 1000 Nights. However, as i said, it can make entering certain combats IMPOSSIBLE, even if you know they are there. So, again, I would caution against raising stealth past 50 ish. You can always have the base stat around 40-50 and toss on a pair of weedwalkers to accomplish most of what you want without completely breaking ambushes.

If you are simply trying to max all the stats to "raise your level" (i.e. health/stamina, since there are no levels), you can increase your health simply by passing time. There is a place in Highcastle that will pass 6 days every time you click on it and you cannot starve to death inside a city. Simply make sure you have enough money to buy rations on the way out and have whatever items you desire to speed recovery, then click away. Going extremely high on health stats will make the game buggy.