The easier route:
* Lots of web sites report aggregate salary information by profession and location. Often these are not to be trusted because {algorithms and marketing} but can still be a useful data point.
* Look for job listings with compensation information in them.
* In-person networking with your peers in other companies. Every decent-size city has meetups for developers and other tech professionals. Not everyone is going to be willing to share what they make with a near-complete stranger (especially if friends are nearby) but some might. This is pretty much the only time I would voluntarily share my earnings with another person besides my wife. There's no reason for me to fib, and unless I'm being profiled for identity theft or something, there are no downsides.
* Virtual networking, a.k.a. join a Slack, Discord, or other kind of forum with people like you and just ask what someone with your experience and skills should make. Since you (probably) don't know any of these people, there is little consequence.
These are all data points to gather and consider in combination in order to arrive at some kind of ballpark figure. You don't have to show any of your research to your employer, but trust me when I say that it's a crucial step because your employer absolutely does already have a very good idea of what you are worth to them. This is data for you to make sure your number is accurate.