The better a pitch sounds, the more careful you should be. Often a victimized person finds themselves being doubly taken to the cleaners. It's really amazing how some people will call you offering a great deal and then steal your money. But my company will get your money back for you. All you have to do is give me your credit card number to cover our low service charge.
Chances are they knew because they perpetrated the scam in the first place.
The best strategy, if you truly think they may be legitimate is to take time to investigate. If it is not a scam, you will be allowed the time to do this.