Soak with vinegar to soften (soak paper towel with vinegar, place wet mess over stains), then scrape with a razor blade (carefully!). For stubborn scales, try something stronger like CLR (the original lime & rust remover, not the soap!), but make sure you rinse really well after. <Neutralizing the residue with a bit of baking soda won't hurt either.>
With hard water like mine, I find there is a fine white haze that develop over a few years on glass that is impossible to get rid of. However, that is not obvious as long as the glass is wet.