Many mechanics and technicians reference websites such as NGK.com and DieselGlowPlug.com to cross reference glow plugs. This is done either by searching the desired part number to cross reference amongst different vehicles or searching by vehicle to cross reference what part number works for the application.
NGK is a globally recognized manufacture of glow plugs. The NGK.com website has a highly comprehensive database, making it easy to enter the model of vehicle in question and search for the appropriate glow plug to use. They also allow for an option to search directly by part number to verify what applications apply. They also provide the option to reverse engineer a part, where a mechanic can find the part number he needs from only knowing specific details of the glow plug such as thread size and voltage.
DieselGlowPlug.com utilizes a cross reference chart where a mechanic can search by the manufacturer brand or part number to find what he needs. This format makes searching for applications based on part number very intuitive. Provide the manufacture part number from the glow plug, and the part number from the site will populate a list of every car that utilizes the plug in question including pertinent information about the vehicles.
In a situation where the part number of the glow plug in question is known, use DieselGlowPlug.com. NGK.com provides more flexibility in situations with limited information.