Hard to say on that one. She is far from correct - that is (as you note yourself) she looks somewhat like a boxer, but a long way from exactly like one - hence you do wonder...
Without going so far as dna profiling to find out, my inclination would be to tend toward the belief that she is probably purebred (though at the same time, I wouldn't risk my life savings in making a bet on it

). The main reason I lean to that opinion is because the first things that seem to go when a pup is mixed (especially mixed with breeds like staffies) is the essential black mask of a boxer. Your puppy still has that, even if its a bit weak - and most mixes don't.
It isn't that easy to tell about her bite from your picture (even though you've got a nice side shot there). So what is it like? Does she have an undershot jaw, or is it more of a scissor bite?