In reality there are so many tell tale signs that the ecu is tuned (the car is faster ;), so it's good idea to flag it in the system when it comes for servicing) and when an expensive repair to be made later under the warranty they could easily send the ecu in question to the ecu manufacturer and they are able to detect any sw changes.
For just normal mechanic; he can use the VCDS to check the boost and he should see the boost right from the idle is different than on the stock tune.