How Do You Know When To Use ' :WaitForChild() ' Instead of just '.'

Hello,

I'm new to scripting and curious on how I should approach trying to decide when it's best to use :WaitForChild() and when it'd be unnecessary. At first I figured I could just use :WaitForChild() for everything but that seems a bit lazy.

Is there like a list of objects that are known to take longer to load, how do I find out about this?