All you need is a scrap of paper to find out whether someone has been inside your hotel room, office or home, says former CIA officer Jason Hanson. Here's how it works: "Take your piece of paper, put it where the hinges are [in the door], you close it as you're leaving," he explains. "So if somebody opens it and it falls to the ground, somebody has clearly invaded my privacy." Watch Jason demonstrate this trick and get more spy-approved safety tips in the video above.