I think there's a distinct difference between the two, and it especially depends on someone's personal level of insecurity and how much they support you. But if you showed your friend a photo, for example, of yourself, and she said, Oh my gosh, you look beautiful! You're glowing! You look amazing! Normal compliment, versus if she looks at it and says, Oh my gosh, you look so beautiful! You clean up nice! Or you look so beautiful, I didn't even recognize you for a second! That would be incredibly backhanded. And I think that's the difference.

I think backhanded is when someone's trying to imply that there's something negative underneath their compliment. So, if someone said, oh my gosh, that outfit makes you look so skinny, that could be implying that they think you don't look skinny the rest of the time, or, um, oh my gosh, your hair up like that makes you look so beautiful, like you're not beautiful the rest of the time, that's backhanded.