Short of doing a chromosome count, I don't think there's a definitive way, but I'd expect the pouch on the hybrid would be darker in color and there's usually more of a yellow "blaze" at the petal juncture on Phrag. d'alessandroi than Phrag. Jersey. Phrag. Jersey also seems to have wider petals than Phrag. d'alessandroi. Maybe Olaf could chime in on this from a taxonomic standpoint