So if i wanna play lead over a song, I need to know if its I IV V etc. those kind of stuff. How do I instantly know? Is practicing the song before we jam it the only way? Or should I memorize all the notes in all the keys so I would instantly know while jamming? thanks
Go to a website that offers it and learn every note, chord, key, scale, etc... It never hurts to know such things.
Well, I usualy go of with finding the root by looking at the fretboard of the bassist/rythm-guitar and then just first play some pentatonic scale stuf.. that's it?

for instance, if the bass player plays C F G D I know that it will probably be in A minor.. it's not really in knowing the notes from your heart, it's just pure logic. If the played notes are in the minor scale of a certain key, then it'll probably be that key

I don't really understand the problem..