In the UK, we have a yearly test (provided that the vehicle is over 3 years old) for road-worthiness called the MOT. It's pretty much a decent indication of what is or what is not legal, although the vehicle is appraised "as presented" and so you could potentially get away with removing slightly iffy mods before the test and replacing them afterwards. You could still get pulled though, as traffic police are notoriously trigger-happy and often badly informed.
Anyway, the tester's manual is all online:
MOT Testers Manual