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Humans carry both bugle (bull) and cow calls, while a bull elk won't make cow noises. So hearing a bugle and then a cow call from the same place is almost guaranteed to be a human.
Humans can be over eager to respond and will bugle back very quickly while elk will take a while.
Elk move around while humans will usually stay in the same spot as they are hoping the elk comes to them.
Bull elk will vary their calls, but humans usually only carry one bugle call and that leads to it sounding pretty much the same.