Salary Analysis & Negotiation

Friends, a genuine question.. need your suggestions...

How can one know if he/she is paid reasonably, or underpaid or overpaid?

I try to analyse my peers network, but I always find people of similar work experiences having similar skills getting salaries higher than me as well as lower than me.. It's kinda distributed..

So how can one evaluate or know the correct salary for a given person? Any idea? I think this will be helpful for everyone during salary negotiations.

Note : I'm a software developer majorly backend with a slight hold of frontend. Also have done a few projects on data science. Around 4-5 years of work experience.