How do you judge skill level and wage for self?

I don't. Just demand more. Greed is good.

On the operations side, I have an answer on how to identify if you are underpaid/average/overpaid. If your CTC is less than Experience x 2, if multiplied by 3 you are avg, by 4 you are overpaid and you will be satisfied at this stage. However, this logic may be different if you are software engineer.

By leverage, determined by what multiple of outcome you are able to produce with the investment the employer makes in you. It not just depends on your own skill and capabilities but also in the ecosystem. A software engineer at Google is paid 3 lakhs a month because he can bring massive revenue to the company with even a simple development. The same person cannot drive that kind of outcome in TCS, so he will be paid 60k per month. If the same person becomes a clerk in a bank, he will be paid 30k per month. Each of these is fair pay in the given ecosystem. So when evaluating your own worth, you need to figure out by what factor you would be able to amplify your wage. https://nesslabs.com/high-leverage-activities

always underpaid

The real answer is whatever you're willing to get paid for your work and time, and if it overlaps whatever budget is being offered and the kind of work it is. Market rates don't matter when you need a job badly and one is being offered to you. When you have options, you can afford to wait for the right offer/better pay. But when you're desperate, you take what you get.

I have a generic formula here: TC (cash component) = 10 * YOE ± max(5, YOE)