I'm a Software Engineer and I think I'm very good at it, at least much better than my colleagues so recently I have asked my boss for a raise (I named a concrete number). He told me that he thinks that too but he cannot prove it. I also told him that I was virtually mentoring one of our new colleagues and taught him a lot. Also in this case he said he cannot know how much I helped my colleague so he needs to think about whether he can approve the raise.
I wasn't prepared for that (because I thought my work is visible enough) and I couldn't reason with him. But I don't give up so easily (I really think I should get a raise) so I already told him that I'm going to have another appointment with him where we discuss how we can approach this issue and avoid such lack of clarity in future.
Next time, however, I'd like to be better prepared and bring my own suggestions - also as a signal that I care.
So my question is: How can I undoubtedly show that I'm worth the money? What criteria should I suggest that would allow him to notice and value my work more?
(I like the company where I work so looking for a new job would be the last resort solution - I'd rather first try to convince my boss that he should pay me more - also because it's a great exercise at diplomacy)