Like others, I agree you should do a measured mileage check.
Unlike others, I wouldn't wait until you've ridden a three figure distance on an unknown situation. I'd recommmend brimming the tank, then ride about 50 miles, and refill.
At a 50 mile distance, even if you learn you have a major issue and truly terrible consumption (say 25mpg), you will still have gas in the tank when you top up.
At 100-120miles, you could be pushing it for the last quarter of the way. So it's just a case of erring on the side of caution.
But the main point is to determine what your baseline figure is.
There are a number of factors which will determine this. For insatance, a 100lb rider, who starts the bike and rides off, should expect a lower consumption rate than a a 250lb rider, who lets the bike run and warm up for several minutes before riding off. Then there are temperature/gas mixture/terrain/riding style/Etc. factors.
I can say that as someone on the wrong side of 200lbs, living in Florida (with the 10% Ethanol mixture), flat terrain, and hot temperatures, I average around 48mpg, riding almost flat out for the short distances between the red lights, where I seem to spend about 1/3 of my time.
If I get onto the open road and can cruise at 60-65mph, I bring this up to well over 60mpg.
I'm happy with that. Your figures may well be different. But first you need to establish what they are.