I can't understand the circumstances, because in the days when I went to functions like 'cocktail' receptions they were formal official events that you couldn't get into unless you were invited and security allowed you in. Going to them wasn't like checking into a hotel or a flight.
Perhaps you mean 'make an appearance'.
If you've been invited to an event you don't really want to go to but feel you must 'show up' however briefly, you might say 'I've got to make an appearance at that reception tonight'.
I agree that I wouldn't call the person manning the door a receptionist, but I have no problem with using check in in this context. Although, when I first read the question, I had a reaction similar to Hermione's, because I didn't think of cocktail receptions as having check ins. But in the less formal and probably more common case of a cocktail reception at a conference, a corporate event, or something of this nature, I can easily imagine checking in by giving my name to someone sitting at a table near the entrance.