#1
I hear this is something that you should make you return a guitar, what exactly does that mean, and how do I pick it out?

Just by looking at it?
#2
it's when the neck isn't parallel with the body.. if it's visibly noticeable then definitely return it!
#4
hmm...

you can use something that's perfectly straight and align it with the neck and see
if it is in angle with the item...

not sure if it'll work 100% though...

#5
Get a metre long ruler.. a proper metal one if you can.. and put it between the D and G strings on the nut.. and on the bridge.

Then get a spirit messure and hold it against the side of the ruler.

That's the only way I can think of checking it.
#6
if its not noticible,,its not bent enough to be dangerous, I just stick the guitar between my legs and look straight down the neck from the head stock down, easy to see a bend then