I've only used it to the extent of telling them that I should bring the card by to show them the signature, not mentioning the police, but I would tell them there might be a serious problem with someone else signing their name and they should look into it. Then of course I set the appointment.
I just thought of adding the police part in when I read the question. I thought it would add even more urgency or get the truth out of them!
You personally go out to see clients?