You could fight against the gym yourself to try to see, and it would gain a little bit of prestige. I would assume it is using the old stats based on when it was placed in the gym. If it is low enough in the gym that you can use pokemon that are half its CP to reach it, then you could tell based on the ammount of prestige you gain.
Another slightly similar situation, if I change the name of one of my pokemon after placing it in a gym, it still has the old name when I look at the gym or fight it.
