Truck Repair: will not crank for @ 2hrs once truck is turned off, side fender, selonoid

Question
How do I know if glow plugs are working? I have a 88 Ford 350 7.3 diesel non-power stroke/turbo. When I turn the truck off it want crank, does that have something to do with the glow pluggs being bad? While its running its fine no problems no leaks at all. How do I know if the glow plugs are heating up?

Answer
use a test light and see if they are powering up. if the truck won't turn over when the engine is hot check your selonoid for starter it should be mounted on firewall or on inside of the passenger side fender. if it's good I would check the starter. it may be failing when it heats up