High voltage definetely burns out those glow plugs. Might take more than a few seconds but it will do it. Replaced a whole lot of glow plugs over the years where operator used a booster of some sort that exceeded normal voltage.
Not all glow plugs are created equal as they change them up over the years. Some more sensitive than others. Some will actually blow the tips right open. Others just pop internally. Then you have the tips that get thin due to leaking injection nozzles so they are more prone to blow easily.
Hard to say how much voltage is sent to the glow plugs because that depends on condition of battery and all wires and connections. If battery is low, putting on high charge the voltage may not even rise to high enough voltage to hurt anything.
Then the other thing. When you start one of those tractors on it's own power, no booster, the normal voltage to glow plugs before you engage starter will be right near 12 volts up to maybe 16.6 if everything is perfect. Keep them held on while cranking and voltage will drop below 11 volts. Keep holding them on after starting, generator starts charging and is limited to about 35 amps. Glow plugs will draw 60 or 70 amps so voltage never even reaches regulated voltage.
Now, if you release the glow plugs for a while with it running, voltage should rise to about 14.5 volts. So, now you again apply glow plugs they will receive higher than normal battery voltage for a period until system stabilizes again to point where the 35 amp generator voltage is closer to normal battery voltage.
So, to be safe, if need to hold them when running, reduce engine speed, maybe even turn on lights to lower voltage, then you will be safe to hold them on . My method.
And as far as testing glow plugs. One other poster recommended the way I also do it. Disconnect all but one and hook one at a time. Good one will show near half way on meter. Two together will show full meter so meter will not tell you if you have one or two burned out.