they will usually make a clunking noise or a popping sound,you will also notice a slight vibration sometimes,and you see a lot of wear on the tires that shouldn't be there,you can also jack the car up and move the wheel back and forth on it,any shop should be able to diagnose a bad ball joint for you,if one shop says its bad and the other says it isn't id try one more shop,chances are the shop that told you it was bad is right,when you replace them,have both sides done ,always do these in pairs,it saves a lot of problems later on with the car,and there not that expensive to buy and have put in,good luck with it.
