As a Patriots fan the easiest way to detect bandwagons is to ask them who the best non-Brady player in franchise history is. You can even give them hints if they need them… if they can’t come up with the name “John Hannah” at all they’re a fake Pats fan.
For clarification if they list a bunch of names and John Hannah is one of them, they get a pass. If they’ve literally never heard of John Hannah then they don’t