If you don't get a good look at the beast, you can usually tell the difference between them by the way they move.
The roe moves in a smooth elegant run with the odd gracefull bound to clear obstacles. It's head is up.
The munty looks uncomfortable and runs head down in a sort of "wild pig" trot.
This looks a wierd explanation even to me but I hope you'll see what I'm aiming at when you next see them.