Pressure testing isn't commonly done, normally a diesel or industrial thing as opposed to the standard gas engine. Essentially it's like flow testing by bolting the head down to a plate and testing not only the chambers, but the water ports, oil passageways and intake and exhaust ports. Done one by one, time consuming and not cheap. Essentially filling the head passages with pressure and waiting to see if any psi bleeds off. Sonic testing can do it much faster which is why pressure testing isn't done much these days.