We know for a fact that she has yet to activate Dragon Force on her own...
...
...sorry, the thought of that froze me for a second...
Anyway, I'll put this out - unless you have reached the Dragon Force state/stage, you are not a True Dragon Slayer.
Corny, I know, but it's probably the best indicator we have for some type of distinction. ...which doesn't help because Natsu was getting sick long before the Tower of Heaven arc, but meh...
I only say it because Natsu has achieved Dragon Force twice that we've seen. Sting and Rouge would have had to achieve it already. Which means that Gajeel must have achieved it during his traininig...which leaves Wendy having not reached Dragon Force yet.