There are quite a few ways to do that. For instance, if someone was a talented guitar player, you could say, "wow, you're really good at guitar!" Or you could say "I'm impressed, you're so talented!" Try to be specific with your compliment and it will be taken as more genuine or real. Instead of "good job," you could say "you did a good job with that presentation, you came across as confident and appeared to really know what you were talking about!"