If the spots seem fast-growing I would suspect a bacterial problem.
If individual spots stay roughly the same size but there seem to be more of them over time, I would suspect a fungus.
If the spots are not getting bigger, and the number of spots is not increasing, I would suspect mechanical damage (no treatment needed)
These are rough generalizations, though.
Look for the link to fungicides & bactericides near the bottom of this page for remedies:
Orchid Pests, Orchid Diseases