Suppose your friend or colleague said, "I can speak English well". This "well" is, of course, very subjective. To properly assess the level of one's language command I offer people this joke. They should read it without a dictionary and laugh in the end.
If they understand all of it and laugh, their level (according to the CEF Levels) is (Pre-)Intermediate. (as far as Grammar and Vocabulary are concerned).
An old man goes to the Wizard to ask him if he can remove a "Curse" he has been living with for the last 40 years. The Wizard says "maybe, but you will have to tell me the exact words that were used to put the curse on you." The old man says without hesitation "I now pronounce you man and wife".