When you watch a boxing match you'll notice the fighters extending their forward arm (left arm for a righty & vice versa) toward their opponent's face without it having the speed of a punch. That is their way of measuring the distance between them.

I can't speak for boxing, but when fighting, it's best to keep back a bit and wait for the opponent to throw the first punch. Most of the time they will throw a punch and pull their arm back. That is the moment to step forward and punch, but remember to not pull your punch back. Always focus on punching past the target.

well, you learn your range on the bags and with padwork. but yea, the jab is your range finder in an actual fight. sometimes you'll see people like ali or roy jones jab and leave it almost fully extended to set up range for a right hand. otherwise if you're more defense oriented, wait for his first punch and slip it. then you know you're close enough, or at least free to move closer while he resets himself

+4 You know by accumulating thousands of hours of sparring and bagwork. Like in any sport you learn to know your range for each kind of punch by practise, practise then more practise. After a certain length of time it becomes so familiar that you don't have to think about it, you know without thinking and your brain works it out almost automatically. . Some fighters do use the jab ar a range finder but for me I have never found this to be effective because there is inevitably a gap of time between landing a jab and a follow up punch. Your opponents head will usually have moved during this gap. Also a jab will often require your arm to be at full extension whereas a follow up hook for example would not have quite the same distance / range on it. . Generally speaking it's usually a matter of keeping slightly out of your opponents punching range, stepping in to throw and then retreating back out of range again. It's the step in that gets you within landing range. . A jab for me is a means of distraction for keeping your opponent from setting up his own power punches. Ali, Roy Jones and others sometimes take this to extremes by leaving their glove on their opponents face or forehead. It is virtually impossible to throw a meaningful punch if you have somebodies glove in your face.

Well you get a general idea by practicing (sparring) with lots of different people. When you are in a fight, you let them throw a few so you can get an idea of their reach versus yours. Ideally you want to get to know your opponent and their strategy before you just go throwing wild punches.