[drc]

You can provide supporting evidence. A letter from your dad, the specifics of the hat versus what McGwire would wear (size, et), and any photos (even if style matches) you can find.

What you can try and do is get a reputable and well known auction house or dealer to sell it. A potential buyer will be more confident in bidding/buying/authenticity if the auction house is a known entity versus some guy they've never heard of. A place like MEARS might take it on consignment and sell it saying it comes with a letter of provenance from the fan who got it.

One thing an auction house might do is say it's game used (by their opinion/assessment) and "attributed to his last game as an A'," with the attribution being the letter from your day. Just a thought.