they leak at the flanges where they bolt to the engine.
generlaly you get tiny pin holes. Go to home depot and buy some fittign adn you can make a testing rig. Can one end of teh water fitting and hokoup a leakdown guage on the othe rend. then presureize teh thign and see if it leaks out.
youcan also us teh same rig witout the leakdown and just presureize it up to like 50-75psi abnd submerge the header in a tank of water. then look for bubbles. if its on the flange it can prolly be fixxed. Just make sure that when youa re done baoting you A. flush teh header jackest with fresh water. B. drain them after flushing so h2o (fresh/brackish) isn't sitting in the jackets.
i'll post a pic of the tesing rig i made later on.