Test the waters of the job market. See what your fair value is there. Negotiate with your bosses to see how much they are willing to pay to not lose you to that job.
Test the Waters
How much are you worth on the open market? If you don't know that, then you can't know how much your boss values you compared to what you're worth, only compared to what you're making now. And that isn't very useful information, because it doesn't tell you if your boss actually values you, only how badly he may be undervaluing you currently.
Once you know how much someone else is willing to pay, you can start negotiating with your bosses. If you know you have an offer waiting for 130% of your current salary, it is a lot easier to walk in and say, "I would like to discuss my salary, I think I am being underpaid." Something like:
Over the past year I have accomplished A, B, and C. On the open market these skills are very much in demand, and I hope that we can negotiate a pay increase in line with my contributions.
If you are really interested in seeing how much your company values you next to the market, you can be a bit more blunt and say:
Over the past year I have accomplished A, B, and C. I have recently been offered a position paying X, much more than my current salary. I would like to stay here, but that is a significant gap in earning potential. What can we do about this?
Be Prepared to Walk
If you aren't willing to leave your job over money, then you shouldn't have the discussion. At the end of the day if your company says, "We value you at Y" which is dramatically less than the X you were offered on the open market, then you are going to lose happiness in your current job as you know you are being paid less than you're worth.