Even if it was by how they spoke Arabic, and not some visual cue.
Probably by the way they speak Hebrew. Of course, I think that they'd just try to avoid any contact, because at the moment they'd actually have to speak to Israelis, their Arabic accent would come through. I noticed this when talking to one of my Israeli friends, he kept saying "Chamas" while I pronounce it "Hamas" with as a voiceless pharyngeal fricative (I'd had to look that one up, hah).