How do you know if a number is a composite number?

Answer 1

There is no simple answer. You need to check for divisibility by every number from 2 up to its square root. There are some simple rules for checking for divisibility by small numbers. As soon as you have found one such factor you know that it is a composite number.

The fact that it is so difficult to tell is why composite numbers made up of two large primes are used for data encryption.

Answer 2

It has more than two factors.