GWX Play question

[Agrajag]

Okay, re-installed the full game last night after about a year away from it. I also installed GWX and pushed out from Kiel to start my new career.

I'm dealing with having to re-learn the sim again on top of GWX but am a bit confused on a couple of points.

GWX, so far, is sending me a ton of radio messages and they're a bit confusing. First, it's 1939. What can I attack and what can't I attack?

Second, it appears that GWX does not let you know that a target is an enemy target. That's fine but what's the best process for identifying something as a valid target? I was looking at a merchant ship last night and still couldn't readily identify it (I think it was a Danish ship) when it got to 1500m directly in front of me (it was night.)

Thanks.