How do i figure out if a number is irrational or not?

Rational numbers terminate or repeat. For example

Rational numbers: 
3.444444... (4 repeats on forever, therefore this number is rational)
5.7 (The decimal terminates after 7, therefore this number is rational)

Irrational numbers:
3.14159265358979323846(The number continues on in an unpredictable way, and doesn't terminate or repeat at any point. Therefore this number is irrational)
Hope this helps!

Any decimal with a repeat bar on top of it is rational but a great example of an irrational number is pi, where the decimal number is going on and on with a mix of numbers ... so yeah