Working with multiple tons of material that dries out as you move it around is hard. There are a lot of steps between the concrete being mixed and when it finally reaches the pour.

Cutting out a piece of a slab and sending it to a lab is for post-pour validation in serious construction. There are pre-pour tests that are much simpler depending on the seriousness of what you’re building.

The slump test is rather simple, for example: https://en.wikipedia.org/wiki/Concrete_slump_test

It’s basically a cone with handles and a procedure that’s easy to learn.