1. Document first
Write down the symptoms, prevent further damage if that's safely possible, and pull together whatever history exists on the windows.
2. Look at the house as a whole
Frame square, sill condition, flashing detail, the material and glass chosen, trim restoration, and which direction a wall faces the sun all shape the result as much as the window unit does.
3. Get a written scope
Spell out prep, protection measures, exact materials, access needs, who pulls permits, testing, cleanup, finish restoration, exclusions, and the change-order process.
4. Compare the reasoning, not just the price
The total matters less than understanding why two providers scoped the same job differently — check credentials on both.
5. Confirm the closeout
Make sure testing happened, the site is clean, you've got documentation and warranty paperwork, and you know what upkeep is yours going forward.