I have found that LED's of my pattern ones have a more transparent shade of green than do 'originals'.
Also, as msinger noted, the labeling differs, on my pattern ones there is no pretence, they are labelled "compatible with PH3 intell....".
The power button can also be trickier to operate but aside from the latter I have had no 'problems' with them.
It might also be the case that, if left too long with out charging, pattern batteries will not lock out the connection to the power terminals as it seems the originals may do but it isn't something I am inclined to deliberately test