Where are the Ability Check rules?

I have recently acquired some pristine 2nd Ed AD&D hardbacks (they really are lovely). One thing I've noticed is that it mentions in both the Players Handbook and the Dungeon Master Guide that you can use ability checks like a Saving Throw as appropriate. However, I can't find a single place in either book where it describes how to perform such a check. Presumably it's a case of rolling a d20 and getting lower than the ability in question. The slightly odd thing about that though is that this is mentioned under the Saving Throw rules, and Saving Throws are roll high. Don't you just hate it when an important rule is left out?!