It’s early in the evening and there’s a knock on the door. You answer and are greeted by an official-looking man who claims he needs to see your utility bill to confirm you’re getting your energy rebate.

Do you let him in?

While he may be legitimate, he may also be using deception to sell you something you don’t want. Here are some suggestions for finding out:

• Ask for a business card. Then, check if it has an address, phone number and website. If the salesperson...