The rule of thumb I was taught was one club for every ten miles per hour of wind into you. As for knowing the wind speed, it’s just practice. For me, if it’s feeling windy, it’s probably 10mph. If I’m raising my voice to talk to others, it’s about 20, if flags are ironed out flat, pushing 30, etc.
At my level of golfing skill, there’s no point in getting much more granular than that, and I don’t know how anyone is accurate to 1 mph without an anomometer.