technique taught me was to use an aerosol can of carb cleaner and spray it along where the carb (this was a while back…) connects to the manifold and where the manifold connects to the head or heads. If there is a leak the engine will react as it gets the carb cleaner through the leak instead of air.
Now the Boxster engine is a bit of a problem in that some notorious air leaks are not found at or with the intake system proper.
For instance, there's the good old AOS which as it transitions from good to outright no good can have a stage of failure that is essentially an intake air leak. The AOS passes too much low pressure to the crankcase and this results in an excess of air getting fed to the intake *after the MAF* as the connection from the AOS to the intake is at a hose connection located directly downstream from the throttle body.
If the AOS is intact and functioning properly other air leak sources have been the oil filler tube. This can crack or split and when the crankcase is under low pressure -- it is under low pressure most of the time -- and can allow air into the crankcase and thus on into the intake via the hose that runs between the AOS and the intake.
A more subtle leak can be the oil filler tube cap. This is a particularly insidious leak -- I speak from direct experience -- as it exists only at low engine speeds. The amount of air that manages to get past the cap is large relatively to the air needs of the engine at idle but small relative to the air needs of the engine at higher RPMs. As I've mentioned before it was an oil filler tube cap leak that I misdiagnosed as a MAF problem and replaced a perfectly good MAF when all that was needed was a $15 cap.
The cap is an easy thing to eliminate. When the engine is running and is at the same temperature as when the symptoms of an intake leak are present or would be present one can try to move the cap about -- while still screwed down tight -- and see if he can get a reaction from the engine, possibly even hear the air rushing past the cap's seal.
The oil filler tube is harder to eliminate as a source of an intake leak. Usually a leak is spotted visually. The car has be lifted in the air so a through inspection -- using a mirror on a stick if necessary -- can be made of the tube. Sometimes a leak of this item is highlight with an oil stain.
There are various vacuum hoses that can and will over time age and possibly at some point crack or split. Generally these fail where the hose slips over its connector spigot at the bottom of the hose. When engine work is done if these hoses are disturbed they can crack/split at this point. Afterwards, the hose tends to want to return to its position so this can work to close the split or crack and make it hard to see, but of course it will make itself known when the engine is running.
These hoses are checked visually with a bright light and possibly even a mirror on a stick with the engine off. Run each one from one connection to another feeling the hose giving the hose a gentle flex looking for, feeling for a crack or split.
With other engines I've run the hoses with the engine idling but these were engines with good access from the top. For a Boxster engine this is probably a job best left to a professional tech, though I believe he'd probably attempt to ID a possible bad hose with the engine off.
(I can also add that if a hose was suspected I'd just replace the hose. I kept a good stock of vacuum hoses at home and would just cut a new hose of the proper size to length and remove the old hose and install the new hose. Course this may not be practical if the hose is formed/shaped rather than just a plain old piece of hose held in place with hold downs.)
A very subtle intake air leak can happen upstream of the MAF say at the air box. While the air is introduced ahead of the MAF and nominally metered by the MAF air flow from the leak is not as laminar as it would be otherwise and the MAF requires a degree of laminar air flow to properly function and the MAF doesn't properly measure the amount of air flowing into the engine. Most often symptoms from non-laminar air flow arise from the use of aftermarket air boxes/filter systems but a stock system that has failed or been disturbed, in some way compromised due to work in this area can also create non-laminar air flow past/through the MAF. The MAF output can be chaotic and strongly point to a bad MAF, a natural assumption, but of course a new MAF doesn't help.