Yes just identify them then use multimeter on diode test or continuity, testing each of the three in turn. Each should measure the same but you may need to isolate whilst testing. Note I've added an image above which shows how they are wired.
As I type I've thought of another possibility. Poor connection between ignition switch and warning light. Here the lower than normal volts from battery put a reverse current through the warning light bulb. You could jury rig a feed to warning light bulb direct from battery to prove.
Probably not relevant to your problem but for completeness the bulb glowing brighter with engine revs issue (mentioned above) is usually down to poor battery connection. Here we have lower than normal volts to warning light from ignition switch but higher volts from the diode trio so again current flows in reverse through warning light, increasing as alternator puts out greater volts with increased revs.
