How do you identify items without paying 100gp?

I can cast the spell identify, but for some reason it still costs me 100gp to cast it which seems weird.

Is there is a skill that allows characters to identify items automatically, if so what skill is that?

Its annoying as have potions galore and no idea what they do and dont want to spend 100gp every time.

The 100 gp charge is part of the spell. Weird, yes. If you have a Bard in your party, I believe they have a much easier time identifying items, as they get a boost to their check (Bardic knowledge).

Is that a feat or is part of the bard build.

Well, the 100 gp cost of casting the spell is part of the rules in the 3.5 sourcebooks, so ToEE is actually the only game faithful to the tabletop RPG that way.
Also, bardic knowledge can't identify items either in this game.

Frankly the fact that ToEE follows blindly the 3.5 rules no matter how well (or badly) they work in a solo video game is the main point of it.

If you have a character who can craft either wands or wondrous items, and you use the Co8 mod, the most cost-efficient method to identify items is to craft either a wand of identify (you need a wizard/bard with craft wand) or a helm of something(don't recall the name atm, but you can find it in the list of wondrous items to craft).
The helm only allows you to identify scrolls and potions, but it's very useful if you don't want to pay 100 gp to identify a potion of cure minor wounds...

Actually, it turns out that (at least with Circle of Eight modpack 5.9.2 NC) you can use Read Magic to identify potions as well as scrolls. It says nothing about it in the spell description, but there's no surprise there - they are unreliable at best.

I can confirm that Read Magic works on both potions and scrolls, without the Circle of Eight mod.

It's also possible to create a Wand of Identify without the mod.. though I'm not sure whether the gold/XP cost is the same or not.