There is one simple and easy method you can use to spot an engineer. Unfortunately it does involve hanging around public toilets. As a consequence of this Billothewisp recommends that you only indulge in this bit of "I Spy" when you are actually taking a leak yourself. Otherwise who knows what might happen.
Billothewisp cannot be held responsible for any over zealous engineer spotting and certainly will not post bail for any person using this blog post as an excuse for hanging around public toilets.
Anyway, the simple and easy method:
Watch the target as he/she enters the public toilet. Observe closely both the pre and post urinary ablutions.
If they wash their hands after taking a leak then they are probably nice clean fastidious people. Some engineers are. Some engineers are not. This is no indicator as to any engineering vocation.
If however, they wash their hands before taking a leak then they are guaranteed 100% bona fide engineer.
You know it makes sense.