Look for any areas where the fretboard chipped away when the original frets were pulled. Check that the frets are the same size as originals, that's a dead giveaway, if you have jumbos and it came with vintage smalls. Tooling marks on the edge of the board and finish removed from the edge is a good tell, although it could be extensive leveling, crowning, and cleaning the edges if they got sharp from the wood contracting. Myself and other good techs can usually tell, just because of familiarity. An experienced local luthier may shed some light. It can be little things, like knowing how Fender did their fret ends, and yours are more rounded or flat, or less angled. It's not just one thing that can tell you.