Pbehn,
Another it depends. Answer lays in the 8-15nm range. F-16 nose on maybe a bit less than 8, Flanker (Su-27) or Tomcat a bit outside of 15 if more towards the beam (side view). Clouds, engine smoke, background, sun angle, aspect (how much of their aircraft they are showing).
Modern fighters have Target Designator Boxes (TD box) that has your designated target in it (as long as someone has a sensor on him). The box can be square, diamond or circle depending on what's tracking him. The above ranges work especially if he's in a TD box. You may have to bring them in (visual acquisition ranges) a mile or two if you only have a general direction and altitude to go by.
When Chuck Yeager spoke of seeing guys at 50 miles he was talking about a squadron or greater sized gaggle. Even with a TD box I could just see a KC-10 at 50 miles with great conditions and I had 2015+ vision.
It also takes training to pick out guys at distance. I still practice it when flying airliners and am usually the first guy to get tally ho.
Cheers,
Biff