I have to admit the mixture check is one thing I probably do badly.
I have been flying powered aeroplanes for 23 years or so. This is the first time that I have come across a mixture check (really!). It is not mentioned in the POH or checklist of any piston aeroplane I have ever flown. To make absolutely sure, I even looked into the POH of the Pa44 (the type on which I mostly instruct) right now: No "mixture check" to be found.
So I would say that you can't do something badly that is not defined anywhere...