Ron Guth: BEWARE OF ALTERED COINS WITH REMOVED MINTMARKS
Obverse diagnostic for a genuine coin (circulation strikes only): large, slanted die polish mark in the right foot of the R in LIBERTY.
Reverse diagnostic for a genuine coin: short, horizontal dash inside the lower left part of the triangle next to the eagle's right leg.
Sources and/or recommended reading:
"Setting Standards" by Skip Fazzari, THE NUMISMATIST, March 2009, p. 99