Question 1 — What's the overall shape?

• Fan or rounded pair of halves → bivalve (scallop, clam, ark, cockle)
• Spiral with a single opening → gastropod (whelk, conch, tulip, olive)
• Flat disc with a five-petal pattern → sand dollar (echinoderm)
• Tube-shaped → tusk shell (scaphopod)

Question 2 — Where does the aperture open?

For gastropods, hold the shell with the spire up and the opening facing you. Opening on the left = left-handed (sinistral) — very rare; almost always a lightning whelk. Opening on the right = right-handed (dextral) — everything else.

Question 3 — What sculpture do you see?

• Radiating ribs across a fan → scallop
• Long straight hinge with many small teeth → ark
• Prominent shoulder knobs on a spiral → knobbed whelk or juvenile horse conch
• Ornate frilled ridges (varices) → murex
• Smooth polished surface → olive or tulip
• Flared lip with a small notch near the front → true conch

With those three answers you're rarely more than one page of a field guide away from a confident ID.