If I don't clearly see a ball out on my side of the court, I can't make a call and it is in. You can't call a ball out if you "think" it might have been out but you couldn't tell for sure.
Now, if your opponent has a clear look at the ball and you don't for whatever reason (they hit down the line and have clear view while you are on opposite side of court, you dive for a ball fall down and don't see the ball land, etc.) you can ask your opponent what they saw and they are on the honor system to tell you the truth, but ultimately it is still your call to make and if you didn't see the ball out and they don't tell you it was clearly out, the ball is in.
Personally, if my opponent is unsighted or doesn't have a clear look, I'm not going to volunteer that a ball was out unless it is very clearly so and I know they didn't have a look and are unable to make the correct call.