Finding an old propane tank when you clear out the garage can be kind of interesting, in the euphemistic sense of the word. On one hand, you potentially have extra fuel, depending on what's still in the tank. On the other hand, you don't know what condition the tank or fuel is in -- and that can be dangerous. Rather than assume that the tank is usable or not, bring it into a propane supply company and have the tank inspected for wear and rust, and then see what you can do with the tank.
Retrofit With an Overfilling Prevention Device
If the tank is empty but in great condition with no leaks (you'll see advice to dump soapy water on the tank and look for bubbles as evidence of a leak; don't do this yourself as you can still miss tiny signs), see if you can have it retrofitted with an overfilling protection device. Small tanks are now required to have these devices, or you won't be allowed to refill them. Not all old tanks can have these devices added, but it's worth asking about because if you can have the device added, you'll have a usable tank.
Exchange for a New Tank
Of course, you could just exchange the old tank for a new one. The size may vary (e.g., you may bring in one big tank and get two smaller ones instead, depending on current inventory at the shop you go to), but you'll have a brand new tank that doesn't have any problems like cracks or rust.
Empty the Propane (With Help) and Recycle
A third option is to empty the propane from the old tank -- have the propane company do this, and never attempt it yourself -- and recycle the tank without getting a new one to take back. You can always buy a new tank later on. If you're not planning to use any propane for a while, storing a tank on your property (this is assuming you're not keeping propane around for emergency purposes) just leads to another cycle of forgetting you have it and then having to take it into a supply company to have the tank checked out. It may be better for you to recycle this older tank and then just wait to purchase another one until you really need it.
Propane needs to be handled carefully, and a supply company for propane is the best candidate for that. These companies truly want you and other propane users to be safe, and they will help you ensure that old propane tanks aren't going to cause you a problem. To learn more, visit a business such as APOLLO PROPANE INC.