In Skyrim the player's character levels up and the player's gear offers varying levels of protection/damage, and in many instances the spells and the magic weapons refer to stuff like "will turn all enemies level 12 or lower etc. etc"
But from everything I can see, there is no way to know what an enemy's level is, at any point! So how do I know what any of these numbers mean? I understand that I can just use trial and error, and I guess the argument can be made that this makes it more "realistic" but then why mix and match these metaphors.
Best Answer
The game levels around you but when you end up seeing is different types of creatures (Draugr Thralls at level 1 for example and Draugr Wights at level 13) instead of the same creatures of different levels (a thrall that is level 1 or level 13). Kind of building off of what Zig-Zag has said, you simply need to learn them. If you do not want to use an outside source or the console commands (if available to you on the PC), then all you have is Trial and Error.
However, if you want to look this information up then I would point you to the UESP Skyrim Bestiary. This contains links to all the major types of creatures and on their specific pages you are given level information so you can see what names of creatures your spells will be able to affect. It is honestly too much information to copy and paste here but maybe someone will amass a table of it all (despite SE not having good table support).