According to this checklist, a User Story should be:

Indepedent (of all others)

Negociable (not a specific contract for features)

Valuable (or vertical)

Estimable (to a good approximation)

Small (so as to fit within an iteration)

Testable (in principle, even if there isn't a test for it yet)

Source(s):

1. Glossary: INVEST - Agile Alliance
2. INVEST at XP 1-2-3 by Bill Wake