Here is a reason to make sure your Keg is purged and the CO2 is as pure as you can get - when alcohol oxidizes it becomes acetaldehyde. Just saying.
Analysis of a beer for acetaldehyde would remove doubt. Why do I say this? Some yeasts produce apple esters. Not green apple, but ripe red Apple. We think apple=acetaldehyde, and that is not always true.
A test would say how much acetaldehyde is there. All beer has acetaldehyde, the goal is for it to be below human threshold.