Your car's gas tank (properly called the "fuel tank"), a most drivers are acutely aware, holds gas. The fuel gauge on the dashboard tells you how much fuel is in the tank. Typically, the gas tank is located towards the center of the vehicle in a protected location.
On most compacts and sedans, this is in between the vehicle's frame, underneath the back seat (or behind the front seats if a 2-seater). On trucks and SUVs it is typically on the frame on the side opposite where the exhaust pipes run and roughly at the center of the vehicle.
The reason the tank is located centrally is two-fold: first, it equalizes weight and negates the effect a full, half-full, or empty tank will have on the vehicle's handling characteristics. Further, it protects it from damage in an accident.
How To Tell If Your Gas Tank Needs Repair
Most fuel tanks do not have problems for the life of the vehicle. If, however, water has been introduced to the tank or the vehicle has sat in one place for too long (with a tank that wasn't full), then problems can occur. Usually the problems will manifest themselves through the fuel filter which will need frequent repair.
The Fuel Filter
If your vehicle's fuel filter requires frequent replacement or the vehicle's fuel system is having a hard time getting fuel into the engine, then you might have a problem with the gas tank. If the problem is frequently plugged filters, then the tank probably has a lot of residue or rust in it that is clogging the fuel lines. If the problem is getting fuel to the engine properly (and the filters are fine), then it's likely that your gas tank has a leak or hole in it that is not allowing a slight vacuum to form; the vacuum is essential to modern fuel pump systems.
Obviously, the smell of gasoline or the presence of a puddle indicating a leak is a definite sign that you have a problem in your gas tank or fuel lines.
Gas Tank Quick Fixes
Rarely will a "quick fix" option work to repair your fuel tank. If it's leaking, but not badly, there are some putty or sealant products that can be put over the leak to repair it. Some work better than others, but few will be a permanent solution. A leaking gas tank is likely to begin to crack at some point, in a way similar to your windshield cracking - think of the leak point (small hole) as a rock chip that will eventually spread.
For most fuel or air leak problems in a fuel tank, the gas tank will need to be removed from the vehicle, cleaned out, the leak(s) repaired, and the tank repainted and/or resealed before being put back onto the vehicle. It is something that many competent mechanics can do at home and is a common DIY project for those who restore classic and vintage vehicles.