A litmus test, if you don’t know, more or less is a question you pose to someone wherein their answer reveals their level of insight into that subject. For these purposes, the question would almost always be “What do you think of _____?”
For example, I think the most relevant one would be Brock Purdy. There are a wide range of nuanced and not-so-nuanced opinions you can have about Brock Purdy that more or less lets you know how seriously someone engages with the NFL (or at least, their evaluation of Brock Purdy).
Personally, mine is Jadeveon Clowney.
in case it needs to be said, don’t interrogate people on their fandom, don’t be a gatekeeper, football isn’t all that important and people aren’t inferior based on what they think about football players, etc. think of it as a mental exercise, “what player inspires a wide variety of opinions?”