Thanks for all the responses. There are several different opinions put forth here that make good sense, but from other angles than each other.
I'm thinking you become a crossdresser when it's pretty easy for almost anyone to look at you and immediately notice you're wearing ladies clothing. Ladies jeans might be OK, but when they're flared and have cute stitching, you're probably outing yourself to anyone who takes the time to notice.
When I get called "maam" from the back, I usually realize I've pushed the envelope too far, but I also get a little thrill from it!