Compare like-for-like scope
A surface-only quote is different from a complete deck structure. Ask whether foundations, framing, railings, stairs, demolition and disposal are included.
Look at the site
Height, slope, equipment access and work at the house connection can change the work involved. A small but complex deck may need more detail than a larger straightforward layout.
Keep allowances visible
Permit fees, engineering and concealed repairs should be identified rather than assumed. A clear proposal is more useful than an unexplained price per square foot.