Language, accents, dress,and mannerisms as well as, frankly, their looks in general. I don’t know what alerted the lady who had the scary experience, but I study people at the gate. I’m not sure if the airline would permit a look at the passenger list, but it’s certainly worth asking.
Check out the pilots when boarding. I always speak with them before finding my seat. Thinking about asking “Are you both Christian?” on my next flight!