Do not bury a moisture problem under a new finish
Brown marks, peeling paint or sagging material should be evaluated before cosmetic work begins. A freshly smoothed ceiling can fail again if the source above it has not been corrected. The ceiling contractor may not be the trade that repairs a roof or plumbing issue, but the scope should acknowledge what must happen first.
In otherwise sound rooms, focus on protection and containment. Covering vents, cabinets, floors and furniture is especially important when sanding is part of the refinishing process.
Clarify which coatings are included
Joint compound and repaired drywall usually need proper priming before the final ceiling paint. Ask whether stain-blocking primer is needed in repaired areas and whether the contractor is pricing one finish coat or a complete painting system.
For older texture with unknown composition, discuss testing before disturbance. Do not treat a previous leak, a dark spot or an old-looking texture as proof of what material is present.