How do you validate a primitive?

I know that this is a SUPER basic question, but because it’s so basic I can never seem to find a quick answer, so I’m hoping this can serve as that.

I usually wind up putting this app down for a few months at a time, and having to relearn it as I come back. This is always a sticking point for me. The rest I can usually figure out tapping around the different menus, but can I get a quick and easy reference guide for myself and anyone else who’s confused? Even the website just says to validate primitives, but not how to.

Thanks in advance!

You should have a dialog box on your screen when a primitive is selected: