A lot of things
There's a mottled quality to these fake Thorpes that is pretty easy to spot. Real Mendelsohns have a nice smooth glossy stock that doesn't look like that at all.
The other big giveaway is that with most M101-4/5 fakes, the black box around the picture is much heavier and also there is noticeably more white space between the box and the picture than there is on the real cards.
Note the real cards below for comparison:
|