So, you can slip into your favorite skinny jeans. But can you do two sets of 15 push-ups with 15 seconds of rest in between? "Thinness doesn't necessarily indicate fitness," says Michael George, a fitness trainer in Los Angeles. You're fit if you can pass four tests, according to the American Council on Exercise (ACE). First, a 12- to 14-minute mile run, to prove your cardiovascular ability. Second, two sets of 15 modified push-ups and 20 leg lunges with a breather in between; this measures muscle stamina. The third test gauges core strength: "Lie facedown, resting on your forearms, then push up onto your toes, so your body's like a board," says Cedric X. Bryant, the ACE's chief exercise physiologist. "Hold the pose for 30 seconds. If you sag, that's not good." Finally, to show flexibility, you should be able to touch your toes. "People need to view the body like a car," Bryant says. "You want a good engine and fuel pump, and good, well- oiled working parts."