It is so easy to check glow plugs. The shay tree way, and it works just fine, take a 12V headlight and make a terminal to put the headlight between the glow plug and the headlight. Turne the key on, and if the light lights up good, you have a good glow plug. If the light is dim or doesn't light at all, the glow plug is bad. The best way is buying a test light that tests ground. But it sounds more like you need to adjust the valves. A Merecedes diesel will smoke really bad when the valves are tight, and start bad too. That no compression thing play a kay roll.