The easiest way is to reach around to the opening in the back and feel water running down from the valve when flushed..(On most models, it's located just between the hinges for the seat lid.) The cheapest fix it is to put in a by-pass. That's the hardest part..You need to remove it to a table bench to replace the valve..Barring that, You're faced with a new unit replacement cost..It's a often sad reminder that we forgot ..that, when winter, comes to do a thoroughly intensive winterization. Happened to me some years ago..
Once you get it fixed..come this fall, after you drained the system completely..I suggest blow the system out with compressed air..you then put a plastic drink bottle upside down in the neck of ur toilet..(keep a string around it~!!)
WBCCI 5292 AIR 807
New England Unit