How do I know if my GMAT score is valid?

The GMAT scores is relative. It varies from college to college. The better way is to check your score in the college you are interested in and compare your score there.
You should also know the range of accepted scores for the last entering class and whether the program have a minimum score requirement.
Your score is one of the component ,but surely the most important component.
Highlight your extra curricular activities and achievements.
Make sure you write about your residence and scholarships.