I do not know anyway to determine this from inside the cockpit unless
you have ground distance covered and fuel flow data on the panel...
Best thing is to work this out before hand... Use your GPS (wonderful
research instrument) for distance measurements... Take off with one
test tank carefully filled to a specific mark, using the non test
tank... Pick the best range setting from the book, pick a start point
on the ground, switch to the filled tank at the start point, and make a
square pattern run (4 sides) for five to ten minutes per side, switch
off the test tank when again at the start point... Land... Fill the
test tank... Do your calculations... Now you have a data point to start
from... Pick a slightly higher or lower power setting and repeat the
test... Now you will know whether you are improving or not, and adjust
your next run accordingly... After three or four runs you will have a
good idea where the peak of the range curve occurs... Of course, a
different altitude and temperature and wing loading will change it a
few percent, but you will be as close as you can get in the real world
if you ever have to do it for real... Besides, this testing gives you
a reason to go flying other than the $100 hamburger run...