How to know what enchantment you'll get in Minecraft?

At the moment not without adding mods to your game but Mojang are currently working on an addition to the vanilla game that will let you see at least the first enchant you will get on your item when you try to enchant it. This has been released in the latest snapshots for people to try out but should be in the next full release.

What "Jo" said, however i do remember an old way back in beta days where if you replaced the language file that had the enchanting text with the normal English text that it would display the enchanting table in English instead of glyphs, however i don't remember how or if it still works, it would be worth a try if you can find out how.