just a quick question: how do you see if the neck is straight or damaged/curved when buying a used guitar? I've never done it. Can you see it by looking at it or do you need a ruler or something else? thanks
Usually you can look straight down the neck from the body side. Usually you want the neck completely straight or ever so slightly curved forward (toward the front of the guitar). Curved backward is usually bad or curved too much forward will make the action high and hard to play, unless you are playing slide.
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You can adjust the truss rod and make the neck straighter if it's too curved.

It can be measured by pressing down the first and 12th frets of the low E string and measuring the gap between the string and the 7th fret. There should be a small gap. If there is no gap, the neck needs more relief. If the gap is too big, the neck needs less relief.

I would say too much relief is better than too little because a bit high action is still playable but too little relief causes fretbuzz on the lower frets.
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For twisting, back bow etc by looking down the neck as suggested. I check neck relief by pressing the string down simultaneously at the first and body frets. It then acts as a straight edge, and you look at the gap between the string and the fret at the 6th/7th fret. It should be very small, about the thickness of a business card, no more. I do this on the bass and treble side, to judge the evenness of the neck bow.