I think this wouldn't be so hard if you are in the valley (or other large tech cities in the US), or targeting large companies (Glassdoor). But how do you do this if you are in a different situation?
In addition to Glassdoor, try data.jobsintech.io (H1B) or angel.co (startups) for salary information. Go on indeed.com, and look for companies around your area to see if any are in those dbs.
After a while, you should start seeing some common ranges between regular software engineers and senior engineers. Use those as a starting point in negotiations. Interview at enough places (10+), and you should see what your market value is.
After a while, you should start seeing some common ranges between regular software engineers and senior engineers. Use those as a starting point in negotiations. Interview at enough places (10+), and you should see what your market value is.