The power of Inspect Element. This is exactly how I found out I was underpaid[1]. A company I worked for used a software called erecruit to manage my contracts. When you click on a clients name, it makes an ajax request to fetch the data. Being a web developer, I inspected the data returned.

I'm pretty sure all the developer did was:

    echo json_encode($queryResult);

I saw how much I was getting paid vs how much they were charging clients. I quickly changed my prices after that.

[1]: https://idiallo.com/blog/how-much-do-you-charge-for-your-wor...