MB specifies that 1 ohm or less indicates a good glow plug. However, you can do an amp draw test on the glow plugs. This is the recommended method on later model diesels and the one I prefer. The amp draw test will tell you exactly what the glow plug is doing. You will need an amp meter capable of handling at least 30 amps. What you will be reading is the initial draw when you connect the meter to the glow plug which, should be somwhere between 17-22 amps. After that the amperage should begin to decrease. I usually let drop between 5-10 amps before I disconnect.
If the initial draw is around 10 amps for example, that would indicate a faulty glow plug.
Reaming the chambers is definintely recommended. This cleans the carbon deposits away from the glow plug. A 6mm allen socket works just fine too.
92 Volvo 740 Wagon aka "Mutt mover"