You go to an RTA certified engineer to have the car inspected. THey look over the car, write down what the mods are, and that they are compliant with ADR... if they're not, they'll tell you to fix 'em and come back...
With the certificate, you get a 2nd copy. one must go to the RTA... make a copy of the other and keep it in your car at all times, and keep the other safely locked away just in case...
If you dont report the mods, policy is void basically.... how leneint they are; i dont know...