Modern Power supplies are pretty good and will tend to have a PF of 0.95 or higher (don't believe that 99.9% PF marketing crap).
That's why independent testing such as this
shows that AX1200 as having 0.99PF above 600W right?
you likely can get it under 5A and thus can fit 3 rigs per circuit.
No, breakers for circuits are only rated for 80% continuous load, so you need to get below 4.26A, otherwise you'll get trips while mining.
To measure watts you need a more sophisticated meter. You need a "wattmeter" which measures both amperage and voltage simultaneously and continually. Your power meter installed by the power companies does this. A handheld meter which can universally measure wattage on any circuit is expensive (usually $500+) but dedicated meters you wire into a circuit or sub panel are much cheaper (say ~$150).
Kill-A-Watts can measure Power Factor (and thus VA and Watts). You dont need a super-expensive device to do so.