I put up a cheat for it but cba finding it.
Basically zoom out as far as you can so you can still see everything on the map in detail (so colonies still look like colonies and not yellow blobs). Then pan the map slowly, but quick enough so everything on the map doesn't have time to load (all you will see is the background of the map, if you keep scrolling and don't stop). UFO's don't need a loading time so they will stick out when it's the only visible thing on the map.
I got 50 ufo achievement in around 500 ticks this way on a 2 tick world. Could have been a lot quicker if I'd actually dedicated the era to looking for ufos viewtopic.php?f=411&t=23677
Found it. This will explain it in a lot more detail with pictures.