Inflate the system to a fairly high pressure. Dunlop systems will take 100psi, but that may not be the same for others. Then, other than obvious visible damage which is unlikely if the system is at least maintaining some pressure, it's really a matter of checking all the pipes, pipe unions, airbags and any other parts that contain pressurised air, for leaks with soapy water or similar proprietary solutions. You might find it's just a pipe union that needs nipping up, but it could be a seal in the union or elsewhere, or deterioration or damage. The only way forward is to find the leak (or leaks, hopefully only one) and fix them one at a time, methodically, until the problem is resolved.
If the air lines are plastic, very gently heating them whilst refitting into a union - if that appears necessary - will help them conform and seal better. If no obvious leaks can be found, another possible solution recommended by Dunlop is to inflate the system to maximum pressure in warm weather or inside a warm environment, and leave it like that undisturbed for several days, again to allow all the plastic pipe connections to fully conform to the connectors.
