The simple way to check the GPs is just to measure the resistance across them (look for less than 1 ohm), they usually fail open (infinite resistance).

If you really want to verify their performance, try putting 12V across them and watch them heat up (carefully). They should glow very bright yellow within a couple of seconds. I have done this by using jumper cables from my battery, hold the threaded part of the GP in one cable and touch the contact with the other cable. Be very careful, they take a while to cool off.