If you stop the point immediately, and they say that they got it in one bounce, you lose the point, as it's their call to make on themselves.
If they agree with you, then good, but unfortunately you can't make the call for them.
You can say something after the point like, "Did you really get that one?" But if it happens once, I wouldn't let it bother you too much, because then it's too late, and if they were sure it was 2 bounces they most likely would call it against themselves. If they are either not sure, or don't agree with you, there's nothing you can do anyway.
If it happens a couple times, and there is an official present, call the official to court to watch some of the match. if not, there's not too much that can be done except to say something like "I thought that was 2 bounces. I just want to make sure you know it's your responsibiltiy to call against yourself" or something liek that.