Who is actually doing the work
Ask whether the person quoting is the person doing the job. At a large company the answer is usually no, and the person who sees your house has different incentives from the person who sends the invoice.
What is your license number
It should come back immediately and without defensiveness, because it is public information. Ours is M-44741. You can verify it with the state board in about a minute.
What would make this cost more
A good answer sounds like: if we open it and find the drain arm is galvanized, that adds this much. A bad answer is silence followed by a larger invoice.
Is repair an option
For a lot of faults it is. If the answer to everything is replacement, that is worth noticing. A thermocouple is not a water heater and a cartridge is not a faucet.
What happens if something is wrong afterwards
With a small shop the answer is that you call the same number and the same person comes back. That is not a guarantee document, it is just how it works when the license on the job belongs to the person holding the wrench.
Questions about any of this on your own property? (940) 231-5706 reaches Norman Alambar directly, Texas Master Plumber M-44741, working Krugerville and seventy-five towns across North Texas.
Speak to the owner
Tell me what it is doing and I will tell you what it takes to fix it.