a light meter is the sure way to tell what percentage light is being allowed in. i think it should be 70%. i don't know who would have these but the police might carry them and eventually because of the popularity of tinting they might add this test to the mot, though not just yet.
Your safest option will be checking that with your local Transport Department, as laws regarding how dark car window tint can differ depending on a region. Also, your local car window tinting shop should be familiar with the law so they should be able to advise you on what kind of tint is legal for your car.