I'm absolutely no expert and prepared to be shot down in flames, but I think polkas are generally 2/4 (ie emphasis on just one beat in the bar) and hornpipes are 4/4 - often written straight but played dotted. If you're writing down from hearing tunes, don't forget there will often be UP beats before the first bars of A or B music (the time taken out of the last bar) - just to confuse the unwitting musician like me who hasn't done grade 5 theory!

Then of course there are 3/2 hornpipes, but that's another story ...