How can you use mental math to determine if a ratio is simplified?

This is a very simple question; don't be intimidated. Just see if you can divide both sides of the ratio by the same number and still result in a whole number on both sides. If not, then the ratio is already simplified. It is easy to figure this out by seeing if one or more of the numbers are prime numbers. 
Example: 5:10 can be simplified further because both numbers are divisible by 5. 5/5 and 10/5 result in 1 and 2. Therefore, 5:10 can be simplified to be 1:2. However, a number like 9/11 cannot be simplified further, because 11 is a prime number.
Hope this helped you!