I will overrule if:
1) I am 100% the ball was inside the line. For balls that appear to be on the line or catching the back of the line I will not overrule my partner if they have the "better" view. The reason why I stick to this is because balls inside of the line can be called from almost anywhere on the court with almost 100% accuracy as long as you're looking at the ball. For balls that are "close" (on the line; back of the line; side of the line, ect.) you can be "mistaken" if you're at a bad angle.
2) If there is a mark contrary to the out call and I see the ball actually make the mark. I do this even on hard courts, not just on clay. If the ball is on the opposite side of the court and I can see that it left a mark I will go and take a look. If the ball is called "out" but the mark shows "on the line" upon closer inspection I will reverse the call.
I have a tendency to play balls that most people would call out (which is why some people refuse to play with me, lol) and every once in a while our opponents will ask me if I agree with my partners call. I havnt had to overrule any of those yet, but I have had to confirm the call a handful of times even though I have no obligation to do so.
"In the 1980's two men dominated--sometimes each other, most of the time everyone else."