A 12 volt test light is a very handy tool to have when working on any car but it is particularly important if you have a old diesel. This tool will allow you to quickly determine if there is power getting to your glow plugs, and if not where the power is breaking down.
For the price this is the best tester Kent has used. It has a long heavy duty and very flexible wire, a good solid heavy duty clamp, and a sharp tip that can penetrate thick wire insulation.
Kent has both manuals and videos on how to test you glow plug system. If you are not familair with automotive electrical testing be sure to check out his on demand video listing in the related products.