I had a few close calls growing up when I was still at home, maybe family members were suspicious but they never "caught me in the act" so there wasn't enough proof so to say.
Eventually I did get caught though. The last gg girlfriend I had found out about me. We weren't living together at this point in our relationship, but we would spend a day or two at each others' place from time to time just because. So since I still had my own place, I dressed up pretty often if we weren't spending time together for whatever reason. On this particular day, my gf was using my computer and found pictures of me on the bed in lingerie and stockings from a recent dressing session. I was actually driving to work when she must've came across them, because she called me on my drive in to work that morning. She did not beat around the bush about it, when I answered the phone hello? she went right into "why are they pictures of you on your computer of you wearing stockings and panties?"
Needless to say, I did the first u-turn I could manage and sped right back to my place. When I got back home, I sat her down on the couch and of course the natural questions followed: "are you gay?" "how long have you been doing this?" "did I catch something from you?"
I was pretty much caught red-handed, so I had to come clean and own up to it. What I managed to maneuver out of, was me dating other cd's and men. I had plenty of meetups before she and I started dating, and although I continued to dress while we dated, I did put a pause on meeting anyone else while we were together. Still, she was very clearly not into dating a crossdresser. She was the type of girl who embraced gender roles and enjoyed being the woman in the relationship. So she freaked out pretty bad, going on about how she had no idea how I was gay and that I was lying to her this whole time and whatnot.
I eventually managed to talk her down a little. The relationship did not last much longer after that. To her credit, she tried to be ok with it and had me dress up for her one time, but it didn't work. I couldn't get into it because I knew she was forcing it. And she just was not attracted to me (mostly because in normal mode I'm a regular guy) as an over the top crossdresser with a miniskirt and stocking tops showing, 5" pumps, and big hair.
We broke up soon after that. We had a decent amount of mutual friends, so I was definitely concerned about her sharing my secret with our mutual contacts. As far as I know, she did not.
The best advice I can give to a CD who is in a relationship with an unknowing gg partner is: they will find out eventually. We all think we cover our tracks sufficiently but we all slip up at some point. Maybe it is a subconscious desire to be caught, I don't know. That being said, it is infinitely better for them to find out on your terms than for them to discover it on their own.
Looking back, the deception did seem to offend her more than the crossdressing itself. After all, she did want to see me dress up in person. As for the dating other cd's/men piece, there probably wouldn't have been any reconciling that.
So, times are changing. If you haven't come out to your lady and you genuinely want a relationship with a gg, let her know. You might be surprised at the support you get. If not, you no longer have the stress and fear and paranoia of having your secret discovered.