ANSWER: If you have an existing remote (a.k.a. "transmitter" or "FOB") flip the remote over, there should be an FCC ID on the back. Using the FCC ID on the back of the remote and the amount of buttons on the front of the remote along with the brand of system, you should easily be able to locate the remote you need online by typing this information into any search engine. For example, "Code Alarm 2-Button Remote FCC ID: GOH-MM6-101890". Now if you don't have a remote to your system you'll have to search using the system's brand name and the model number of the system, for example, "Code Alarm CA-4054".
Now if you don't know the model number (don't have the owner's/operator's manual) someone is going to have to drop the dash panel down that's across from your knee's when your sitting in the driver's seat and locate the system's main control module. Most aftermarket systems will be easily found because it's usually held up with a few nylon zip-ties against some factory wiring (you'll be able to tell that it was not factory installed). One the module is located the model number is commonly located right on the top of the module.