You have a quote for a new gas furnace, or you are about to ask for one. Before you commit, the job is to check what is written down. None of it needs special tools. It needs a clipboard and some time at the existing unit.
Start with the age and the planning date
Read the install year on the existing unit's data plate and work out its age. Then read the gas furnace page for its typical service life and how to turn an install year into a planning date. Age is an input, not a verdict. A unit that still runs well may still be the right thing to plan around, and a newer unit with a poor service record may need more attention. Write the planning date you are working from at the top of the quote.
Check the fit before the price
Ask the installer to confirm the cabinet or closet size, the clearance the unit needs, the filter slot, and the flue or vent route. Ask what changes if the new unit is a different size from the old one. Ask whether the gas line or the electrical connection needs any work. A quote that skips these questions has not yet been checked against your house.
Read the written terms as separate papers
A furnace quote often bundles several things: a manufacturer warranty on the unit, a separate workmanship promise from the installer, and sometimes a service plan offered at the same time. These are different papers, so read each one. For each, find who issues it, what it names as covered, what it excludes, and what it requires of you, such as registering the unit or keeping service records. Find any waiting period and the day it starts counting. Ask how cancellation and transfer work if you sell the house. The general terms are explained on this site, and the published figures live in its tables. A salesperson's summary is not the contract.
Ask for the records that go in your binder
Before you sign, ask for the model and serial information in writing, the installer's license details, the start-up record, and a copy of any permit pulled for the work. Keep them with the quote. These are the papers the next owner or the next service visit will need.
Compare quotes on the same list
If you collect more than one quote, lay them side by side on one list: the same unit size, the same vent work, the same warranty language, the same start-up steps, and the same removal of the old unit. A difference in price means little until the scope matches. Compare the written scope, not your impression of the companies.
For the seasonal check of the existing unit before you decide, see the fall furnace walk-through.