[ghayes]

You could also collect every strikeout ball and put them in a bag marked by pitcher for analysis. After a few games, you could quite easily see which had uncharacteristically higher tar on them versus baseline. That is, if the MLB actually cared.

[dreamcompiler]

Assuming strikeouts with no fouls. Which is probably true much of the time with these unhittable pitches.

[peter422]

Pitchers are putting the substance on every ball. Nearly all strikeouts are caught directly by the catcher, and the 3rd strike is the hardest to get, so it would be a very good sample indeed.