In a diesel engine, glow plugs function similarly to spark plugs. While spark plugs create a spark, a glow plug heats up. The heat causes the plugs to glow, and that is where they get the name glow plug. Glow plugs are necessary for starting the vehicle and keeping the pistons moving. It's important to know the sings of bad glow plugs.
Listen for the sound of hard or no starting on cold days. Instead of a steady cranking sound, you will hear it sputter and each sound the engine normally makes will be more obvious. The sound is perhaps the most pronounced and most alarming symptom of bad glow plugs. If the engine requires several tries to crank, it's a symptom of bad glow plugs.
Observe how the car runs while you are driving. If you manage to start your diesel engine, note any unusual behaviours of the car. If it jerks, sputters or sounds louder and less even than normal, your glow plugs may be going bad. The jerking and sputtering can feel like you are low on fuel or be less pronounced such as a the vehicle "skipping" every few minutes.
Look at the glow plugs light on the dashboard, which usually looks like a squiggly line. If the light comes on and off intermittently, take note of when the lights are on and when they are off. Knowing when the light goes on and off can help you figure out what outside forces may be influencing the malfunction of the glow plugs.
If you own a diesel engine, it's a good idea to always have a few back up glow plugs in the glove compartment.