At this point, the only real way to differentiate Florida and Spotted Gars is through identifying where they came from and/or which hatchery they were produced.
I do know that some guys here at MFK have noted that there are some physical differences between the two species, but generally speaking, if it is sold as a Spotted Gar in a Pet shop/Fish supplier, it is likely a Florida Gar (Lepisosteus Platyrhincus).