1. If his name is Ben, Jon (but not John), Noah, Joshua, or Jacob (but not Jake), you're probably on the right track.
Definitely not Jewish are Christopher (in any form, Chris, Kris, etc.), Anthony, Kevin, Timothy, Nicholas (Nick, too) any initials (P.J, A.J), or any Jr's,
2. If he's wearing a Star of David or Chai necklace.
Keep a look out for the cross necklace.
3. If he has excessive tattoos, your Zayde might rip his skin off.
4. If your mutual friends are all Jewish and they don't know each other.
Or if they all went to the same Hebrew school or summer camp.