Hello: One test is to wait until no hot water has been used for about 1/2 hour. Go to the heater and feel both inlet and outlet pipes on top. If the inlet is cold, water must be running into the tank. Normally it would be warm. If the outlet a few feet away from the tank is as hot as right next to the tank, hot water is likely flowing as well.
Another test is to look at the meter, (if you’re on city water) and see if there is movement. If there is, water is being used.
One last test is to feel the line running from the relief valve on top or upper side of the water heater. That may be running. If the line is hot, follow it to it’s end and see if there is water or a happy banana plant growing!
What sort of plumbing do you have, (copper or steel) and where is it (crawl space, under slab)?
Let us know what you find out and we’ll start putting together a plan… and asking more questions 😎