I don't agree with the idea that you need to know how good you are in order to get better. If you want to improve here's some steps that I think will help:
1) Find a definition of 'good' and make it your goal to achieve that, find parses of high level players of your class in high level content, or find a player in that category who's willing to share their performance numbers
2) Compare yourself to high level performance and see how you stack up
3) Study some guides on your class, watch youtube videos of high level players and study their key presses and their situation, you should study each fight very carefully and take notes about what they do and when
4) Do the same fight on the same class and repeat what you did in step 3. Record the fight and rewatch it, and compare what you're doing to the previous notes you took on what the high level player is doing
5) Use the comparison to make changes to what you're doing
6) Once you've put in this effort to self-improve, then seek help from a high-level mentor of your class and ask them to give you advice on how to improve even further
7) Take very detailed on on the changes you need to make, then master each of those changes one-at-a-time. Only focus on 1-2 things at once and simply do them over and over until they've become 2nd nature, then move onto the next change and master that.

It doesn't really matter if you're 'good' or not, if you do these things I believe you'll walk away happy that you did.