I think my parents know because of a few instances, all from my teenage years, but I really can't be sure.
1. When I was 14 (or thereabouts), my Mum found a Pampers in my chest while searching for something else. I persuaded her it was one of my friends playing a joke, but I'm now not sure she bought it.
2. I once left a Goodnite under a pile of pillows in my room while I was in the bath and came back to find it wasn't there. Now I know my Step-Dad had been in my room collecting any glasses I had in there and moaning about it, so I suspect he found it. Later, I found the Goodnite in my room under something else. Now either I hid it somewhere different to what I thought, or my Step-Dad put it back. I'm not sure.
3. When I was on holiday with the folks in Sri Lanka, I bought some nappies while I was there as they were cheap. My parents had gone on a day trip to somewhere I wasn't interested in, so I figured I'd hang round the hotel all day in a nappy. I went to get a massage, with shorts over the top of my nappy, but I was horrified to find that the masseuse expected me to strip naked and get under a towell. I thought 'well, disabled people might want massages as well' and I just stripped down to my nappy and got the massage. Now later my parents asked me what I was wearing when I got the massage. I said nothing of course. Again, I can't be sure what happened. Either they were told, or maybe saw me, or they were joking about me being naked. I'm not sure.
4. The general internet history on the family PC. I was always careful to delete the history after every time I went on nappy-based sites, but I remember a row I had with my Step-Dad over how long I'd been on the internet (I was limited to two hours per session at the time). He then showed me the page where he could check exactly how long I'd been on. To my horror, it listed sites with minutes surfed on next to it. I then quickly admitted I'd been on longer, even though I hadn't, to avoid the possibility that the history of those websites hadn't been deleted from that page. Not sure if he checked, not sure if they were there.