There is only 2 ways to test with the Glow Plugs still in the Engine. With the Ohm meter as you have doneor you can use an amp meter.
However, from what you said it sounds like you have one glow plug not up to snuff.
However, removing them and testing them on the Battery is the more definitive test. But, that does not meet your keeping them in the engine criteria.
If you look funola has some info on testing with an amp meter.
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If you remove them use a glow plug reamer or some alternative method to ream out the Carbon.
The Glow Plug in the pic was OK by the Ohm Meter but as you can see in the pic it is glowing in the middle and not the tip like it shoud so it is no good.