Truck Repair: 350 big cam spewing oil out exhaust, exhaust gasket, air gauge

Question
350 big cam in old k100 leaking oil out exhaust. Turbo was seized so that's been replaced, but oil really pouring out. Thinking its a valve seal, but how is easiest way to test this? Getting ready to pull exhaust mani to find out the cylinder for sure

Answer
Looked it up in my rebuild book only way to test it is by pulling the heads and using a cummins tester. Which im sure is expensive. I would send them off to a machine shop and have them tested and reworked. Do have a idea don't know if it would work and there is no measurement to say what would be acceptable. Why couldn't you rotate the engine so both valves are closed. Take a steel plate with a exhaust gasket and bolt it down. Have a hole with a air gauge and and air chuck. Apply a small amount of pressure to the head and watch the reactio on your gauge. Maybe by doing each cylinder one at a time you could tell a difference between whats leaking and whats not. Just a idea i've never tried it myself.