Well one way to tell would be to find a same generation Passat Wagon with same transmission type and race it... if you win easily then you're either chipped or the other driver is a bad driver LOL. Okay I'll be more serious now, but my point is you should probably be able to tell if the car is chipped or not just by how quick it is compared to a stock VW (unless you have no experience with a stock VW to compare to). I'd also think whoever you bought it from would have mentioned something about it being chipped...
But if not then, yes, one way to tell would be use VAGCOM. If you don't have it I'm sure you can locate someone in your area that has it by putting up a thread on the forums (but try VWVortex because it has way more traffic than PassatWorld:
http://forums.vwvortex.com/forumdisplay.php?510-VAG-COM-Diagnostic-Forum). Logging some data (for example any of the following: boost actual and boost specified values, fuel rail pressure, and timing) will reveal whether the engine is running like a chipped car. In the case of boost values, for example, assuming you don't have any kind of boost leaks or anything then even a stage 1 chipped car will be boosting in the 18-20PSI range whereas a stock motor would be nowhere near that. Another way would be to e-mail all the big tuning companies and ask if they have a record of the car being tuned by them (by supplying the cars VIN). If the car is chipped then chances are fairly high that it was done by one of the following 4 companies: APR, REVO, Unitronic, or GIAC. There are others but those are the bigger players and the more common tunes so there's a decent chance if it is chipped then it was chipped by one of them and they should have a record of it.
