[lsversaw]

If the alarm system didn't get wet, and hasn't been chewed up by a dog, it should be fine. BUT there are electronic components that degrade over time -- capacitors mainly -- so your idea to test it is a good one.

You should be able to test it on a bench by hooking it up a 12V power source (spare battery, cig lighter socket of a working car, etc.). The simple test will be to ignore the Smart Windows, remote start, and the ignition kill wires (if the alarm itself works it would still be worth installing, right?) and just hook up the sensors and alarm horn. To simulate sensors that are just switches (like the door-open sensor), just twist those wires together to simulate a normally-closed switch, or separate them to simulate an normally-open switch. Connect the motion sensor(s) like you normally would and lay the sensor(s) on the bench. Then shake the motion sensors, close the normally-open switches, open the normally-open switches, and wear earplugs in case the alarm horn sounds. (Sorry if you already knew some of that. I try to err on the side of too-much-info.)

Here's some reuseable, tool-free connectors that will help with a bench-top test setup:
"Wago 221-412 LEVER-NUTS 2 Conductor Compact Connectors 10 PK"
Amazon sells 'em. They are perfect for underdash wiring because they eliminate soldering, crimping, and taping. And you can un-connect as easily as they connect. I supply them as speaker connectors for the radios I rebuild.

If you test the alarm and it fails, let us know here and I or others may be able to give you some additional resuscitation tips.