For me, knowing some basic (and common) chord progressions is very helpful. Like if a guitar player is playing a G chord (and especially if I know the Key is G),,, I'm gonna guess that the other two chords in the jam are gonna be a C and a D in some order. Because I know that the most common progressions are I-IV-V - or some combo of that. Like G-C-D,,, or A-D-E (in the key of A),, or C-F-G (key of C),, etc. If the jam is is in G, and there is a "funny" chord in there that is not C or D,,, I'm gonna think - this is kind of bluesy sounding, maybe that funny chord is an E minor, because the VI minor chord is very common in blues sounding songs (also very common in a lot of country). Or maybe it's a B minor, cuz a III minor is pretty common. Look for common pattern in songs. The more you know them, the more you will start to hear them- and then you will stat to hear them all over, recognize them, and be able to play them. ---- ALSO, you can start incorporating those common patterns into the songs you are making up. Like 1-3minor-4-1 would be a great jam, and a common pattern. And if you want to add a chorus on that jam, 1-5-4-1 might work, because it's a common pattern - and it's all works in the key of (what ever). You are making stuff up, but there are rules and conventions to help guide you. I will say, that there are just some songs that don't follow rules, or do their own things - and for those song, sometimes there is just no substitute for knowing the song. I go to open jams sometimes and it drives me nuts when a guy calls a song that has a tone of changes, and stops, and arranged parts... It's a jam!!! Play something people can jam on. For me, that reflects more poorly on them for their choice of song than it does on the players not able to follow them. Good song leaders will say what key, and give heads up for (whatever) might be tricky. They will also do things like flash you numbers if there is a funny change coming or say "here we go to the 2" to let the band know to go to the II chord. You ultimately want to be able to recognize those common patters when you hear them. --- that's the "ear training" thing. My other advice is to learn "standards". What ever that means for you - it doesn't have to mean jazz standards like it used to. It means standard for the kind of music you are into. If you go to jams all the time and guys are always playing Little Wing, then learn Little Wing.