We had a game where the other team kept using those glossy rubber and cork center balls. I don't think they really knew the difference, but we calmly asked the ump to use our baseballs. I did let my guy pitch one inning after we hit one inning with them. Man those balls really kill any chance of getting hits.
this happened to us in a championship game vs. an out of town team. They pulled out 2 balls from a rawlings bucket they bought at dicks, balls looked legit buy were rubber/plastic.