A way to prove that a plant is a hybrid is that it's characters fall outside of it's typical description.
Such as, varying numbers of sepals, petals, and pollen bearing stamens. Flowers not being solitary. Leaves not simple, alternate, with toothed edges. Etc.
http://ntsavanna.com/elm-leave...
Of course there are varieties of the plant, but if they meet the key characters, they are the same species. A hybrid would have to exhibit a differing set of characters.
http://florida.plantatlas.usf....
http://www.levypreserve.org/Pl...
http://keyserver.lucidcentral....
