Courtesy runners

[TERRY1]

I understand courtesy runners ok but had a team yesterday that used the same runner for both pitcher and catcher. I know this player would be an illegal sub. and in rule
3-1-4-a of FED book it "may be discovered by umpire" I had always been lead to belive it was the other teams job to catch this. Should I as a bu call this when I see it and call an out and restick to dug out?