By what criteria can I safely say (and it will be true) that I became a professional in webflow?
I think that depends on whoever you ask. There’s always gonna be someone who’s going to say: “No you’re not a real professional because you don’t do XZY right”. Are you asking because you want to put that skill onto your resume? In the end you have to be comfortable with your skillset and be able to say to yourself: Yes I know the tool well enough to do whatever I need/want to do with it. Also, you’ll never stop learning and there’s always going to be someone who’s better (or worse) than you! Good luck!
If I may give my input, I would say that you can consider yourself as a professional, as @lauraniebel says, when you are confortable with your skillset and the tools you are using.
Webflow is a fantastic tool and as yourself it evolves everyday.
From my really short experience being a professional is also learning and be motivated to improve to best satisfy your client and yourself.
In programming for example it’s probably the biggest part of the job to wonder around to find answers or inspiration for your challenge.