Quote:
Originally Posted by FloatingPoint
Hello!
Can anyone point me in the right direction on how to check if a 1 billion digit long number is prime?

Of what form? Are you interested in knowing how big a number of your form can be checked for primality (or "probable primality"), or do you have special interest in one specific number?
If little/nothing is known about the number's form, you could try trial division to disprove primality. Of course, if you don't find a factor in a month or two, you still have no idea if it's prime...