How do we find vapour pressures, and boiling points of various substances?
How else but by experiment?
And so you get your pots and pans together, or your beakers, and flasks, and test-tubes, put on your safety gear, put a thermometer in the liquid, and let rip with the heat.
Note that the vapour pressures of many common liquids, water, ethanol, mercury, are extensively tabulated at various temperatures. Of course, at their normal boiling points, whatever these are, their vapour pressures are