Just looking at the frame is easier if the said Hybrid or MTB is newer. I find the top tubes on MTBs tend to be shorter than most of the hybrids I have been on. But that being said there are hybrids that are closer to a MTB and there are ones closer to a road bike. Some would have considered my Masi Café Solo a hybrid and others called it a flat bar road bike. But in “general” I have found hybrid frames to be a bit less bulky than MTBs. If they lean towards road bike they tend to have longer chain stays more like a touring bike. What really puts a spoon in the pot is the class of comfort bikes tossed in the mix.