Finding Max and Min on Scientific Calculator

I have a very general question here. I am wondering how to use my calculator to either find the max/min of a given function (Which I doubt I can do with a simple scientific calculator), or to PROVE that a certain set of numbers are in fact maximum/minimum of a given function. I would think that I could only complete the latter of the two on a scientific calculator... But I want to be sure.

Anyways, I have a Casio FX-991ES Plus. The 991ES is the same, pretty much.

Any tips would be great!

Thanks