Is there any way to determine the level of an enemy? I'm trying to figure out what spells will work on what enemy. For instance, the Illusion spell "Mayhem" says "Creatures and people up to level 25 will attack anyone nearby for 60 seconds:" This is a Master level spell, yet it only affects creatures up to level 25. I'm now at level 45. Would this spell be worth it to me. Will I be facing any enemies that are level 25 or lower? Any way to tell? Thanks.

The only enemies that are higher than level 25 - in general - tend to be boss-esque.

So, 20ish% would be considered too high by your current level or something like that.

As far as "taking" spells are concerned, what do you mean? Reading spell tomes?

In any case, why not just take any spell? What's wrong with learning them all?

Oh. I assumed that most of your enemies were around the same level as you. I didn't realize that they kind of maxed out at 25. I guess that's why I was able to up my playing level to Expert. When I get to 50, I'll try legendary.

Yes. Skyrim's leveling differs from Oblivion. There are some enemies(usually unique) that actively level up with you. But for the most part, enemies either have a static level or a set level. Though, the higher your level, the more frequent "boss" enemies will appear. But still, they aren't that common.

Always nice to have bandits fighting their chief (even if they get slaughtered).

Is there a certain limit on how many spells you can learn?

The level of creatures and people affected by Mayhem can be increased by taking http://www.uesp.net/wiki/Skyrim:Mayhem

As many as there are in any given school.