Approaching aircraft - friend or foe?

[Viduka]

Just a few questions regarding taking on the enemy airforces...

For the first time I have 2 flak guns on deck and have been using them to good effect but I have been wondering - how do I know the planes i'm shooting down aren't German?

Is it generally regarded as the done thing to just shoot at anything that flies near me? Or do I need to be waiting to see if they attack you first before you open fire?

Also, I upgraded my conning tower so now I have 2 light flak guns (double turret) instead of one light + one heavy flak.

Which is the best configuration for taking down planes? I'm assuming it's the 2 light flaks, as it cost me reknown to upgrade the conning tower that houses them.