The Comprehensive LaTeX Symbol List", a 178 page PDF. Scan through section 3, "mathematical sysmbols", and eventually find:
Squint at the screen. Zoom in. Yes, it's \cong. Carry on typing. Then you need another symbol...
Surely there's an easier way? Well, I've just stumbled across a marvellous website that gets rid of the need to scan through hundered of pages of tiny symbols. Meet Detexify - LaTeX symbol classifier:
Do what it says -- draw here!
It takes a few seconds, but then a bunch of symbols appears -- including \cong. Perfect!
My symbol drawing is dreadful here, because I'm using a mouse, and using it right-handed at that. But that doesn't matter, because the system is very forgiving and provides a range of possibilities "similar" to the crude sketch:
If you know what it looks like, you can find it.