As judk mentions, an offer in hand (or even a ballpark from the recruiter) is solid data.

Also, Wealthfront keeps their startup compensation tool pretty up to date (https://www.wealthfront.com/tools/startup-salary-equity-comp...), so that's useful.

Agree that Glassdoor's data isn't very reliable. There are too many crossed incentives in that system to make me want to believe.