Testing glow plugs

Hello all,
Been doing some poking around on testing glow plugs and found that you remove them and check resistance or apply 12V and see if they go red.... but I've read that some people test them with a clamp on AMP meter by clamping it over the glow ply lead.... can anyone confirm if this is an accurate way to checking them? My 2003 TD5 doesn't have much room for easily removing them, so hoping the clamp on meter will be accurate enough to test the.

Clamp on amp meters detect the magnetic field in a wire carrying current.
If your glow plug is open circuit, it won't draw any current and won't 'glow' and compared to a good plug will be easily detected.
You'd need to isolate the individual plug from the circuit (I am presuming they are all connected together like on my 300tdi ?) and feed it 12V

Glow plugs draw about 15A initially which ramps down to around 5A in approx 5 seconds.
An isolated glow plug should be around 1.2R (1.2 Ohms) or slightly more, which is easier to measure than Amps .... plugs tend to head towards being open circuit (0R) when they go duff.
If it's less than 1 Ohm, replace it.