air shocks

The reason im saying is because i recently changed my rear shocks and my compressor and wasted almost $2,000 because the rear shock was leaking air and was making my compressor work all the time and burned it so i was just wondering cause the front ones dont look like its connected to the compressor but they do have a plug in on top of it and i was just wondering how i could tell those are wore out cuz they cost almost $400 each and i dont want to change something i dont need to change