most likely experience, upshifting is way easier than down shifting !! down shifting takes time and experience, one thing i note is at what speed to i upshift, that helps with what speed and gear you can downshift to.
as others noted you have to learn the sound of the engine to match rpms engine to transmission. double clutching to get them to match, takes time and practice, been driving my 29 & 30 for a few years and still not good at it. grind them here and there, but getting better :)
main way to know the clutch needs adjustment or going bad is having to put it to the floor to get it to shift/ disengage the transmission. unless you ride the clutch the clutch pads should last awhile. how long depends on driving style.
yes the clutch wears like any clutch. the main adjustment is adjusting the clutch pedal. the details are in the owners & repair manuals.
