I'm Zerg (platinum, near diamond) and I want to know how to interpret my early game scouting against Protoss.

I imagine that I need to know the different timings of build orders, how many Assimilator/Gateway/Cybernetics Core/Robotics Facility are required. I know that there are plenty of different builds but I need to know the main ones.

Globally, how should I react if I see at 5-6 minutes no expand or 0/1/2 Assimilator(s) on expansion?