What checks should i have done on a used LPG car?

You can get a 234$ pre car inspection from most insurance companies and will gurantee the car for 30days if something goes wrong with it if they gave you the go ahead to buy it. Usually when buying LPG you should get someone to check for leaks in the tank and the tank most be no less than 10years if more you need a new tank which will cost you it should be stamped somewhere on the tank when it was made. Take it for a test drive obviously and makesure they is no backfire sometimes it will blow the airbox apart which can be modify but it shouldn't do that if it is running fine.