Spot contact at from high altitude.

[Janosch]

Other methods... depending on server, "enemy external views" and "padlock" may be allowed, but these fall into the cheat category, and I can't confirm that padlock locks on to targets that are a great distance away.

It's also possible that low flying opponents communicate via teamspeak to the high flyers. On some servers, there are even AI flights, and they use the radio too, so if you know their position, you might get some idea as to where the opponents are.

As for filters, the only filter one should disable is the cigarette filter, and cigarettes too in general. And be sure not to overstress your eyes! Trying to find a dot or lump of pixels that's flying over a e.g. a snowy forest or ocean is tiresome, and you may find that it's impossible to focus your eyes properly after awhile.