You don't. It's either an educated guess by means of past experience and knowing average limits of use, or power measured from the wall and accounted for by the average efficiency of the psu. If you pulled 400w from the wall on a 80% efficient psu, that means the psu is running about 320w.
Figure your cpu will pull 65w, the motherboard can pull upto about 100w (generous) and the gpu will pull upto @ 120w. Total pc output is @ 280w ± maxed out. Gaming loads average high at @ 70% of max. So average gaming loads, your pc is somewhere around 200w-230w range.
230w draw is less than the maximum output of the UPS, so whatever full load time is, you'll have slightly longer to shut down before the battery runs out and forces shut-off during power outage. For power spikes, you're pc is less than maximum, so you are covered.