A running toilet doesn't automatically need replacing. Water can escape from the tank into the bowl, or the filling mechanism may fail to stop the incoming water. The plumber needs to find which part is causing the problem.
Kohler's guidance describes several possibilities, including a sticking lever, water entering the overflow and worn tank seals. Hearing water run doesn't tell you which one applies.
Tell the shop whether you hear water continuously or the toilet occasionally starts filling when nobody has flushed it. Water leaking from the tank into the bowl can cause that occasional refill as the tank level drops.
Mention when it started and whether anyone recently worked on the toilet. If there's water on the floor, say that separately. The plumber needs to know about an external leak as well as the running sound.
You don't need to take anything apart before calling. A photo of the toilet and an accessible model marking can help the shop identify it.
“Repair toilet” doesn't tell you much about the work. Ask which part has failed, what's being replaced and how the plumber will check that the running has stopped.
If you've already paid for a repair and the sound has returned, send the invoice. Ask whether this visit is checking the earlier work or investigating a different problem, and whether there's a charge.
The EPA recommends investigating a constantly running toilet or contacting a plumber. You don't have to put up with it as part of normal use.
Ask why. A replacement may be offered for reasons beyond the running, but the quote should explain those reasons rather than treating the sound as proof that the whole toilet is finished.
Compare the repair with the complete replacement: new model, installation, removal and disposal of the old toilet, and anything excluded. The relevant question is what each option fixes and what, if anything, it leaves unresolved.
Before booking, confirm the visit charge. If the company offers both repair and replacement, the quote checklist helps you compare what you're being offered.