the way my mom figured out I was a demi girl was because I was complaining to her about how people think my gender was fake, then she asked why they thought girls were fake, and that was when I realized I made the mistake of telling a story that would make no sense with out the input that I was a demi girl, so I had to tell her
In other words, you should probably tell her through some sort of story casually rather then going up to her randomly or like idk making a sign er whatever. If you don't make a big deal, neither will they.