A quick and easy way to see if your tyre tread exceed the minimum legal tread depth is to take the 20p test. Simply place a 20p coin into the main tread grooves of your tyre. If the outer band of the 20p coin is obscured when it is inserted, then your tread is above the legal limit.
If the outer band of the coin is visible, then your tyres may be illegal and unsafe and should be checked immediately by a qualified tyre professional. When taking the test, remember to check at least three locations around each tyre. As the test is so quick and easy, stay safe by checking your tyres at least once a month.