No doubt there is a method to check how well your valves seal, but I don't know how... when the valve seats are cut,lapped ,the valves are vacuum checked,if ok its assembled.did you bend a valve in 6 cyl. on assembly?as was said how much shiming was used,did you check valve springs and seat pressures.if the heads ok,then head gasket or rngs-worn or broken.timing.