Seems to be a lot of questionable claims on lumen output.
How can the layman determine whether a light they bought is actually putting out the lumens that the manufacturer claimed?
Lumen measurements are very tricky, not easy to get it precise.
So the answer depends on how accurate you want to be.
For approximate comparisons, buy a cheap chinese luxmeter, strap it in a fixed position in your bathroom (assuming that your bathroom has a whitish interior and walls) facing a white wall and point flashlights to the opposite wall, so the luxmeter does not see the projected beam directly (integrating bathroom method). This way you can compare the light output between flashlights. You can calibrate the set-up if you have a flshlight with a known output. Mind that this method is in no way accurate, you are easily 20% off.
I think one of the easiest ways would be to measure (with your multimeter) how much amps particular flashlight draws from the cell. When you have that data you can go here -> LedCalc - luminus flux calculator select appropriate led and get some rough data. You would also have to subtract ~15% as a loss due to driver efficiency, lens transmission loss etc. But you would get usable numbers easily and fast and you probably already have multimeter…
Just bare in mind that when you have light that uses 2 or more cells or light with 2 or more LEDs some calculation is needed to get “amps per LED” data…