Painting quotes are notoriously hard to compare because two contractors can look at the same house and price completely different amounts of work. A written, itemized estimate is what makes the comparison honest, and refusing to provide one tells you something on its own.
First, the surfaces. Not "paint the upstairs" but which rooms, and within each room whether the walls, ceiling, baseboards, casings, doors, and closet interiors are included. Closets and door edges are the two most common quiet exclusions, and both are irritating to discover after the crew leaves.
Second, the preparation. This is where the real money and the real quality live. The estimate should say what happens before the paint: patching, sanding, caulking, degreasing, stain-blocking, priming. If it says nothing about prep, you are not comparing it to a quote that includes a full day of it.
Third and fourth, the product and the number of coats. Brand, line, and sheen for each surface, plus how many finish coats and whether primer is included. "Two coats" over an unprimed dark wall going light is not the same job as two coats over primer, and the difference shows.
Fifth, the schedule: a start date, the number of working days, and what happens if weather moves an exterior. Sixth, how changes get priced, in writing, while the job runs. And seventh, what happens at the end, meaning a walkthrough where you look at the work in real light before the final payment leaves your hands.
An estimate covering those seven points takes a contractor an hour to prepare properly. That hour is the best available preview of how organized they will be once they are inside your house.