Just this past Wednesday in Highland, Indiana, two men gained entry into a residence and stole cash. They claimed to be testing the water for bacteria after an alleged line break.

Once again, a real-life case reminds us of one of the most important rules of scam and fraud prevention: never let anyone inside your house unless you know, beyond reasonable doubt, who they are, why they are there, and what they are doing.

It’s not enough to just believe what they say. You have to verify.

Any time a municipal employee needs access to a residence, they will be carrying identification. Always ask to see it. If you are still unsure, call the department they represent and confirm that someone is supposed to be making visits that require entry. Be polite about it—there’s no need to be combative at this point—but have them wait outside and lock your door while you call.

If they bolt, that’s an obvious sign of a scam in progress. Call the police instead. If they become angry or abusive, that could be a sign, but it could also just mean you’re dealing with a bad employee. Make the call and report the behavior while you’re at it. You don’t have to let anyone in—tell the city to send somebody nice. They work for you.

Be extremely cautious if two people are standing outside your door; a lot of times these crooks work in pairs. However, if there is only one person and you decide to let them in, go ahead and lock the door while they’re in the house. Sometimes a second person is waiting to enter while the resident is distracted.

For obvious reasons, these scams (which are really just robberies) tend to target people who live alone, since they can’t be in two places at once. They also specifically target the elderly, so make sure your friends, neighbors and relatives are aware of the dangers. These crimes can occur anywhere.

Finally, if you’ve let someone in your house and realized your mistake before they’re gone, don’t let them know you’ve caught on. A cornered criminal can be a dangerous object, even though it appears most of these perpetrators are relatively nonviolent. Get a good description of the person, their vehicle and a license plate number, if possible. Wait until they’re gone and call the police once you know you’re safe. Your health is far more important than your possessions or your cash.