If she knew, she never let on - even to herself
When I was growing up in the 60s we had a storage closet where my mother would put things ready to discard or give away, including a lot of her old clothes, including bras and girdles. When I was alone in the house, this was my playground. I am sure I never put things back exactly as they were, but it did not matter that much. If she ever figured it out she never said anything. She was also the type unlikely to admit, even to herself, let alone my father, that anything was "different" about me. She was a great mother, but very old fashioned.
She has been gone for 10 years now, so I'll never get to ask.
My mum knew/ knows for certain
Although I was always very careful as a child wearing her clothes I'm absolutely sure she knew.
I don't know whether she still thinks I dress but I guess she suspects.