How do you test if a Number is Prime?

There is no easy test. You need to divide the number by all the primes up to the square root of the number, so if you want to know if 56789 is prime, you need to divide by all the prime numbers up to 239. If you go through all the primes up to the square root and don't get an even number as the result of the division then the number is prime. (Incidentally, the factors of 56789 are 109 and 521.)