Sometimes yes, sometimes no. It is more frequently a matter of recognizing cultural patterns than anything as simple as facial characteristics. "I can't eat that," or just an unusually long study of the menu may be a hint of keeping kosher. "I've got to get home early, it's Friday," may be a hint of Sabbath observance. The occasional turn of phrase. And, of course, some Jews wear kippot (yarmulkes, little hats) that announce their faith very publically, or wear things like a chai necklace (it looks like the greek letter pi, but is actually the Hebrew word Chet-Yud, pronounced chai, meaning life).