Cross-fertilizations -- like that. Most "ear" people can not help you, because they rely upon something that we tin ear people do not have. Because they get along fine with their ear most never got deep enough in the theory stream to know what is happening, so good luck asking them questions.
The only thing ear players have that tin-ear players don't, is practice listening. I know, I was one of the tin ears. But if you know 2 things, you can go a looong way.
Forget about watching the guitarist. I've had guitarists intentionally turn away. And if they're playing in an altered tuning or with a capo, you're forked.
First, learn how to find the key. Flip on a video of a live show, and play along, and just keep going until you hit on the key. It's easier than you think. I personally like to do it with someone like BB King, because he'll often call out a key, or tell you to start on 1 or 5 or whatever. You can do the same at jam, but it's easier to do it by video. After a while, you can probably learn to hear and identify many keys, especially the common ones. It still gives me a kick to hear something and know what key it's in!
Second, learn the major scale, because damn near everything you play is going to be based on a note in that scale, in whatever key you're playing. And on bass, those notes are always in the same place relative to one another. You're generally going to be playing the 1, the 4, the 5, sometimes the 6 or the 2. When you know where those are, you're golden. And soon, you'll be able to hear not only what degree of the scale you need to be on, but you'll probably be able to anticipate what's going to come next, even in a song you've never heard!
With some practice, you'll learn to hear what you're playing, the same way you learned to talk to others and carry on a conversation!
