How can you check to see if a fraction is simplest form

You ask yourself if the numerator can go into the denominator equally.

Divide both the numerator and denominator by the same number. The simplest form of a fraction is when the fraction cannot be divided anymore because if you do, it'll become a decimal.
For example:
36           18           9            3
--- ÷ 2 =  ÷ 2 = --- ÷ 3 = ---
48           24          12           4
 
So, the simplest form is 3 over 4.