Either really comprehensive tests (that you read) or read it. Usually i find you can skim most of it, but like in core sections like billing or something you gotta really review it. The models still make mistakes.
For even mid-level tasks it will make bad assumptions, like sorting orders or timezone conversions.
Basic stuff really.
You've probably got a load of ticking time bomb bugs if you've just been skimming it.
Lots and lots of tests!