Golf holes are measured from the middle of the tee-box, down the middle of the fairway, to the center of the green. So if a par 4, for instance, had a curve or dogleg in the fairway, the distance as stated above would be longer than measuring a hole "as the crow flies" or from the tee-box directly to the center of the green. Par three holes usually don't have fairways per say, so they are measured as the crow flies since they are designed to be completed in one full shot and two putts.
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Some times they eaves drop on the caddy when he is telling the player. But in recent years they have used things such as skycaddy and other GPS systems.