you mean the glow plug light, theres a couple things you can do to test them witch i will explain later on,
just check the nut on top of the glow plug isnt corroded or missing
quick storie about diesel, we had a diesel van come into work with 'check and repair glow plug warning light not coming on", so were testing the glow plugs, a couple were kaput, then we check the timer relay, that was kaput so we swaped with a test unit
so after 4 hours trying to diagnosing the glow plugs/timer relay/ wiring harness
turned out the globe in the cluster had blown !
electrical = allways check the basics first