Early in the game, you can't waste time to check what the guards inside are. But at the same time you don't want to send your entire army as you'd lose other opportunities... In the campaign I've had guards ranging from a few level 1 troops to a kyron.
So. If dungeons are hard to reach and require some turns how do you tackle them?
- send a sacrificial lamb to explore
- send a troop that should normally beat it on average - but then what's needed?
- ...?

Depends.

In multiplayer, I mostly ignore them for precisely the reason you state: you just don't know what you're getting into, and you might have to disband the rewards even if you win (which means you may then have to clean THEM up).

In the single player (which means campaign, thus far), I'll probably investigate with the hero that ran across it to see whether it looks do-able, and reload if something goes horrifically wrong.

Sometimes I'll send in a cheap scout, preferably a fast one with Night Vision, and explore it very slowly. Go a few hexes per turn, retreating back a (hopefully) safe distance. Enemies will charge when you get close or when you can see them, so you're looking to trigger their 'Lets charge!' behavior without actually being within their range. Make use of corners too, in case there is a Musketeer or Giant lurking in the darkness with a long range attack.

If the enemy is killable, I'll kill it and continue scouting. Once you've got more experience as a player, you can usually get a feel for the danger level of a dungeon after you see the first couple of monsters. For example, if you see a Bone Horror, you can guess there are probably a few more lurking in the darkness. If you see a swordsman, then you're probably looking at a large stack of T1 trash with perhaps a T2 around. If you see a wolf or a boar, you can bet there are several more of them in the darkness.

Dungeons in AoW:1 were one of the many things that drove me crazy about that game and made me greatly appreciate the "hint" of difficulty level shown in exploring similar locations in AoW:2 and AoW:SM...

This is a bit of a cheap gimmick, but if you send a battering ram or other harmless unit at a dungeon with autocombat enabled, then it will show you what's in there, and then automatically retreat.

Also, if you bother to install DevEd (which is a pain that I can't remember to do, but I have a copy sorted out) you can look at the lists of possible defenders. For example, dungeons never have doom horrors as defenders, ruined castles have an entry with two of them plus some wolves and wolf riders, and the other sites never have them actually.

Amusingly the creature lair has four leprechauns+fire sprite as a possibility.

This ^^^ is freaking brilliant. I take back 30% of the things I've said about you.