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Todd Schultzwhoever posted that you should have a known authentic example in hand is spot on. Also, I think it is sometimes a good idea to buy a set of reprints, especially if it's of a set that you would really like to collect and the price isn't too bad. Larry Fritsch (RIP)cards sells several. This won't protect you against counterfeits or all reprints, but it's a good investment, IMO.
The first m101-5 I ever bought was a reprint--George McBride--I did not buy a reprint on purpose.
The next 200 m101-5s I ever bought were all reprints--from Fritsch--I bought these on purpose. I scanned every single reprint and played with how they could be "aged", as well as compared them to scans of known authentics, especially graded. There is no way I will ever be fooled on these again, especially if I have them in hand.