A painting estimate is a description of work, not just a number. When two bids on the same house come back far apart, the gap is almost never the paint. It's what each crew intends to do before the paint, and how much of it they wrote down.
Start with the surfaces. A good estimate names them: walls, ceilings, trim, doors, closets, or siding, soffits, fascia, shutters, porch ceilings. Vagueness here is where scope disputes come from. "Paint the living room" leaves the ceiling and the trim genuinely ambiguous, and you will not find out which reading was correct until the crew is already there.
Then look for the prep, listed as its own work rather than assumed. Washing, scraping, sanding, patching, caulking, and priming should each appear. On an exterior especially, the prep is most of the labor, so an estimate that doesn't mention it is either hiding a low-prep approach or hasn't thought about it.
Check the coats and the product. "Two coats" and "one coat" are very different jobs, particularly on a color change or over a repair. The estimate should name the actual product line, not just a brand, because a builder-grade line and a premium line from the same manufacturer are not the same purchase.
Look for what is excluded. Honest exclusions are a good sign, not a bad one. Rotted trim replacement, unknown repairs behind siding, and lead-safe work on pre-1978 houses are real possibilities, and a contractor who names them up front is a contractor who has thought about your house rather than copying a template.
Finally, check how change orders are handled and when payment is due. Changes should be priced in writing before the work happens, and the last payment should come after you have walked the job. If an estimate covers surfaces, prep, coats, product, exclusions, schedule, and change orders, you can compare it to any other bid fairly, and you already know a fair amount about how the crew works.