I really like 5263's ideas on this - especially the part about explaining to the opponents that the two of you saw it "differently". Excellent. Leaves open the possibility that *either* of you might have been wrong, *and*, still leaves open the possibility that one of yer opponents may have *also* seen it clearly out and would then award the point to you.
I'm just glad I don't have to play on hard courts very often, especially against the younguns that it so damn hard. I hate having to call "likely" out balls good, but I won't cheat - and *do* understand that if I don't know with 100% certainty, then it's "in". Fortunately, at least with new balls, on the hard courts where I'm occasionally forced to play, the fuzz will generally leave a mark with which I can make the "out" call.
As for playing on clay, it strikes me as just common courtesy that I should circle the mark for any and all close calls that I or my partner make. For a first serve, I'll circle it after the point is over (unless it's questioned). There are certainly times when a mark can't be found, but for the vast majority of balls - *if* one is inclined to make only accurate calls - it is *not* hard to find the mark.
If a partner gets upset when I've over-ruled his call and circled the "in" mark, then that's just fine. I now know that he's not someone with whom I have further interest in playing tennis - or even being around. Living by the "silver rule" works out just fine with me.
I play "old man" age group tennis tournaments and it's well known who the cheaters are, and officials are often called out even before the match starts. That must feel great to the cheaters.
Most tend to stop showing up. On the other hand, those of us with a reputation for fairness tend to not have to worry about bad calls coming from the other side.