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A throwback jersey is one that duplicates a jersey worn by a team or player from the past. Real fans tend to purchase throwback jerseys (along side more modern sports gear) because they actually know the history of the team and its players.  On the contrary, bandwagon fans typically purchase team gear that represents the more modern logos, colors, and players. True fans are also more likely to spend big bucks on team gear and throwbacks are usually more authentic and costly. True fans will never boo their team because they are trying to inspire them, not make them feel worse. It's okay to be angry with a less than perfect performance, but making your team feel like trash won't help. Real fans stick with their team for better or for worse. A true fan's loyalty is always to his or her team first. That doesn't mean that they can't support or like other players, but it does mean that their loyalty lies with their team's players first.  For example, it's fine to appreciate great players like Peyton Manning but if you're a New England Patriots fan you support Tom Brady first. Additionally, if you have players who are on your fantasy football team but are not on your favorite football team, you can support them only as far as it doesn't interfere with your favorite team's success.
See if they own a throwback jersey. Notice if they ever boo their team. Determine if they support their team's players more than any others.