How do I test if an integer is composite?

I'm making a math game for a school project. Whats the most efficient way to test if a variable is composite, or not prime?

You could try finding the remainder of every number from 2 to one less than the number with a loop. If the remainder is ever 0 then it's a composite.

Global number num=8175

Global number isComposite=0

Repeat num-3 times

Compare num%(loopindex+2) =0

--- stop loop

--- set isComposite to 1