Online, specially in Syndicate, I find action every sortie. There are certain areas where you are very likely to find a fight. There are also a bunch of bombers coming and going from time to time, that can keep you bussy.
In Syndicate server, if there are enough people online, you can patrol around friendly ships and look for flak bursts tracking enemy planes. If that doesn't work, and you are really itchy for a fight, you can go fly around the enemy ships, at low or medium altitude and the players in the other team will find you soon enough.
I like to grab a 110 with bombs and go visit the enemy bases. The bomber base first. You can get really lucky there when the Wellingtons are landing.
If I can't find bombers I visit the fighter bases and soon enough I find some Hurricane chewing on my rudders.