Some people underestimate their level and some people overestimate their level. We may not really know what a person's language level is just by looking at their profiles but we can go through the disscussions that they joined and the notebooks they wrote.
I think underestimate can show a person's modest attitude towards language learning. If we can't make an accurate assessment for ourselves, perhaps underestimate will be better than overestimate.
Very interesting question.
I personally use the exam results that I have and am preparing for. For example, I have been studying for the CPE (C2 English) for some time and have been passing by a small margin (low C). The problem is that someone can have advanced listening and reading skills but poor speaking skills. So in reality, it's impossible to really know a person's level of proficiency without knowing them a lot. I might have made a basic mistake and some people reading my comment might then think I am not a C2-level student. So no, I don't believe there is any accurate way to know your level unless you've passed a recognized exam (like DELE or DELF), that's the best way I can think of examining someone's level of proficiency and even then it's not perfect.
Its a very difficult problem to resolve because, as others have mentioned, we probably underestimate or overestimate our abilities.