They will hybridize in nature. If you have access to jstor, here's an article JSTOR: An Error Occurred Setting Your User Cookie (there are many other papers on hybridization of north american hylids). It also looks like the hybrid offspring keep some fertility (end of the first paragraph of that article).

Telling them apart is easy with a calling male. You can find sound recordings here: ARMI - Frog and Toad Calls

Otherwise chromosome count or comparison of blood cell sizes (and I think also the cells that make up the 'sticky' bits on the toes) I think are the way to go but not really practical for most people.