what skills (if any) do I need to train to scan for ships of other players?

1) The onboard scanner (the D-scanner) can only be improved with use. There are no skills to train in game, but a familiarity of use, local knowledge and an idea of what the d-scanner tells you are the ways to make it work for you. You can only find a ship with the d-scanner if it is at a celestial (planet, moon, belt, etc.), not if it is in a safe spot or in a mission.

2) Chances with combat probes are improved by the four Astrometrics skills in the science group. If you have all four trained up to lvl 4 then you have a good chance of finding most ships, but again it requires a lot of familiarity with the method and system. You can use a small ship to probe other ships out and then change into a much stronger ship to go and cause trouble, or you can work as a team with a fleet.