Does anything in the play testify to jason's background as a hero?

I really don't think so. The guy is really kind of a jerk and his past heroic exploits were made possible by other people. Really if it wern't for Medea's cleverness, he would have never gotten the Golden Fleece and hence any sort of hero at all. If he had not been considered a "hero", he would have been lucky to marry a sheep herdre's third daughter let alone Creon's royal daughter. When Medea reminds Jason that it was her cunningness that allowed him his stature, he downplays her contribution by saying, "Your cleverness played a part" and "your service did no harm." So no, this guy was never a real hero.