You take a paper, a pen and you write : 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397
:-)
more seriously here is a function to check if a number is a prime or not but beware you can only use it with relatively small numbers as it is not particularly optimized...
public static bool IsPrime(this int number)
{
return (Enumerable.Range(1,number).Where(x => number % x == 0).Count() == 2);
}