I think it would be very difficult to tell. The only reason a therapist might be able to tell when the patient is having transference, is because they go by not only nonverbal clues in session but documents they have on the patient and what the patient has actually told them and so forth.

I think all therapists have countertransference of one sort or another at some point, but the good ones will go home and figure it out and try to stop it before it interferes with therapy in a big way.