Well, if you see the aircraft, you can take it's bearing relative to you. Thiw gives you already a line on a map where the aircraft can be, but not an exact position...
If you take a device to measure angles (I don't know the name in English) and you measure the angle of the aircraft relative to the horizon, then you can calculate a rather exact position as follows...
The measured angle is "a", so an aircraft right above you should have a=90°... If you substract a from 90, you get a value "b" between for an aircraft on top of you of 0°, while the horizon is 90° ....
The only 'gamble' we have to make is the actual FL of the aircraft... But if we take an average of about 12km heigth, the outcome won't be too far off...
The distance the airplane is away from you equals: 12km / tg(b)
So if you measure 45°, the distance is about 12km...
If you measure an angle between ac and horizon of 30°, the distance is: 12 / tg( 60° ) = 20.7 km
Although you don't have an exact result, I think you will be close enough... Anyhow, since we travel at about 13 km/min (1,1km / 5 sec ) , we will be already some distance further as you calculate the result...
Hope to have helped you out...
Tuur