In most cases, mechanical fuel pumps start to leak before they go completely out. This generally shows up as a "clean" spot on the side of the pump where the gas has washed away any dirt, crud, or paint. It can also show up as gas in the oil pan, so don't get insulted if I tell you to go sniff your dipstick (G)!
If I don't see any leaks, before I change a pump I always check the rubber hose that connects the tank to the metal fuel line. These are notorious for drying out and cracking which lets the pump suck air instead of fuel. The symptoms are very similar to a dying pump. That usually means that the car will start OK (maybe with some extra cranking) and will idle and drive OK if you don't crowd it too hard. But when you drive it hard it will buck, spit, die out, etc, and it usually gets worse the faster you drive.
My wife says if I buy another Mustang, she's gonna divorce me. I'm sure gonna miss her....