is there a fool proof way to tell if my flexi joint has a leak? only reason Im asking as i was taking a peek under the car trying to fathom out why the frick both tyres are wearing more on the outside edge when I thought well im down here i will check the exhaust, the entire underside of the flexi joint from the down pipe is covered in what looks like black soot, I would assume if theres no leak then the joint should be clean....or am I wrong?