Noting wrong with the paint method as it does work.
But if you want a easy way to do it go and buy yourself a Ray tek heat indacator.
All you will have to do is hold down the button and find the hot spot on the pipes.
Pretty easy to get it right as the ray tek has a lazer pointer that shows you exactly where the heat is comming from.
Just thought I would share a clean and easy way to do it.
Ray Tek's are about 70-80$ and you can get them at any NAPA autoparts