The ONLY time you should crawl under a car is when it is supported by a jack stand or on a lift. A jack is used to jack the car up and let it down. It is a huge safety concern to use it to support the car while crawling under it. You can kick the tire from the outside of the car if it won't come off, but keep your body parts away from underneath the car. When a jack lets go it does it in a hurry, without notice, and without provocation. Use it only to temporarily suspend the vehicle when needed, but nothing else.
EDIT: As a commenter pointed out, differentiating between a "jack" and a "jack stand" may be beneficial to the uninitiated. To that end, here are some pics and explanation.
Typical scissor jack which is used on a lot of vehicles as OEM equipment from the factory. While scissor jacks vary in size and shape, it should be noticed the small base and head, which do not lend well to stability. They are small so they can be stored in a small space. Other jack types may be used as an OEM for changing tires in an emergency, but they will almost always be small and overall have stability issues. This should not be used to keep the vehicle supported, but rather for lifting and lower the vehicle.
Typical jack stand which is used to support vehicles for a more lengthy amount of time and where safety is a concern. Jack stands will vary in size, shape, and amount they can support, they will almost always have a wide base to make them stable while supporting a vehicle. This doesn't make them perfect, but when used properly, there should be no fear while crawling under a vehicle. Notice, the positive locking support near the head which allows for height adjustablility. As long as this is locked into place correctly, there is very little to fear about failure during use.