H.264 is a codec, Mainconcept and x264 are encoders.
Anyway, I might be wrong but I don't think you can tell which encoder was used. You probably could if you were experienced with the output each encoder produces so you could look at how it was encoded (knowing each encoder has its preferences), but the only thing you can easily tell is whether x264 was used or not (with the user data headers x264 writes), and I think the answer to that is obvious
