I'm sure there's a test you can do with a multi-meter to test the low voltage back from the glow plugs. IIRC it's a matter of taking a low voltage reading on the individual glow plug wires from the glow plug relay under the bonnet.
Should it prove that the glow plugs do need changing it goes without saying to do all four. Number 1, nearest the cam belt end of the engine is the hardest to get at, but it's a matter of a long reach socket with flexible connections and extension bars. I did mean with my 1/4" drive socket set. You'll also need a pair of long reach pliers to get the leads on and off the top of the plugs. Another tip is to put some blue tac into the socket, it helps withdraw the old glow plug after it as finally been unbolted.
Last but not least, I did read on one of the old post to take the rocker cover off to give better access. My advice is DON'T, it made to no difference to accessability plus you've got the aggro of buying a new rocker cover gasket and fitting before putting the cover back on.