How do you tell the difference between rational and irrational numbers?

Rational numbers have are wither self terminating, or if they are non terminating, then they have a pattern. 
Irrational numbers are non terminating and have no pattern.

A rational number is a number that can be written in a ratio of two integers. ex: 8 is rational because it can be written as 8/1, .001 is rational because it can be written as 1/1000.
an irrational number cannot be expressed as a ratio. ex: the number pi (3.145) is is not a simple fraction 
hope it helps :)