First you need to see if there are wear or scratches around the holes where the bolts and washers are to see if they are free of wear. Next you can look at the outside of the fender bracket hole in the outer bead. There is a reinforcing plate behind the hole area that is spot welded to the inner side of the fender bead and should show the dimples on the outside on both sides of the hole in the bead where the spot weld occurred. Repro fenders, even the good ones from Albrecht Towing, don't have the holes or reinforcing plates in them. Also, look at the reinforcing plate where the runningboard bolts are located. Repro fenders reinforcing plates are a little different than original Ford fender reinforcing plates. I have a NOS 28 fender and Albreght Towing repro fenders are very close in appearance to the NOS fender I have. I don't think many people could ever tell the difference. Probably the best way to tell, is find an original Ford fender and compare your questionable NOS fender with a known original Ford fender in the areas I mentioned above.
Rusty Nelson