Walk it and feel for soft spots
Boards that flex or feel spongy underfoot usually mean rot. Pay extra attention to areas that stay shaded and damp, and to boards near the house.
Check railings and stairs
Grab each railing section and push. Movement means loose fasteners or deteriorated framing behind the surface — a safety issue, not a cosmetic one.
Stair treads and stringers deserve the same attention since they take concentrated weight.
Look underneath
The framing, ledger board and fasteners underneath tell you more than the surface does. Rust streaks, gaps at the ledger and cracked joists all warrant a professional look.