One way to tell that an explanation is pseudoscientific rather than scientific is to

The answer to this question would be: how they respond to the evidence of the contrary.

When they are given the evidence of the contrary, pseudoscience mostly will deny all the evidence. When discussing with a real science the opponent mostly still accept a part of the evidence so there is a chance that they proven false (falsifiability)

But the most reliable way is to examine their evidence. Most pseudoscience doesn't have enough evidence or research to support their claim. But this method will need resource and time.