The only deterministic, polynomial-time algorithm for primality testing I know of is the AKS primality test (http://en.wikipedia.org/wiki/AKS_primality_test). However, there are a lot of very good randomized primality tests that are fast and have extremely good probability of success. They usually work by finding whether the number is composite with exponentially good probability, so they’ll either report that the number is composite or will require you to say “maybe” with very good confidence.