Measure the opening and document the connections before choosing a sink, faucet, toilet, or shower fitting.
The short answer
A replacement must fit the existing space, supplies, drain, mounting, surrounding finish, and household use. Ask who confirms compatibility and who supplies the product. Include shutoff condition, removal, installation, testing, disposal, and any permit question in the written scope.
What to notice
Fit is more than outside dimensions
For a faucet, note hole count, spacing, deck thickness, supplies, and accessories. For a toilet, rough-in, flange and floor condition matter. Sinks involve cutouts, mounting, drain position, and cabinetry. A retail listing cannot see the existing conditions. Preserve measurements without removing a working fixture merely to inspect it.
Owner-supplied and provider-supplied products
Ask who handles ordering errors, damaged items, missing parts, returns, and manufacturer warranty. A provider may set different labor terms for owner-supplied equipment. Record the exact model and finish. Avoid scheduling removal before the correct product and required parts are available.
Finish with a performance check
Ask to see hot and cold flow, drainage, controls, seals, and surrounding surfaces under normal use. Check for leaks again after the fixture has been used. Keep care instructions and warranty documents. If the work uncovered a damaged valve, drain, floor, or wall, document whether that follow-up is completed or separate.
One useful question
Should I buy the cheapest fixture before requesting labor quotes?
Price alone does not establish fit, parts availability, performance, or warranty support. First document requirements and ask how owner-supplied products are handled. Compare the complete installed scope, not a fixture price against a labor estimate for a different product.