How can you tell if a person is a programmer?
When you ask them a question there will usually be a discernible pause (long enough to notice by a mere mortal) for mental lexical analysis, pre-processing, linking, syntactic/semantic analysis and optimisation before they answer.

I also noticed that it takes a disproportionately long time to obtain an answer from them to a trivial question such as would you like a cup of tea?, which would leave them hang in an infinite loop until some specified time-out cuts their thinking thread short and they provide a random answer (whatever was previously written in their answer buffer).