I think you could think of something better to say or ask. You could ask some other question before offering to buy the coffee, to which the person should know an answer that would universal among all of us.
The question couldn't be something like "Who is Satoshi Nakamoto?" Instead, it should be something inconspicuous lke something about the weather or the like.
Think Fight Club but more nerdy.
Okay, I think it should be fairly easy and inconspicuous for your average Bitcoin enthusiast to display a Bitcoin logo somewhere, be it on their phone, key ring, etc. Another bitcoiner would initiate an identification verification by saying, "I like your phone/key ring/t-shirt/tattoo. Is it a Satoshi Nakamoto?" and the appropriate response would be, "No, but it is a
bit like one."
Voilà! You've now clandestinely identified yourselves to one another as members of the anarcho-crypto-economic vanguard as well as maybe prompting eavesdroppers to google "Satoshi Nakamoto," wondering where they can acquire this phone/tattoo/pair of alpaca socks.
