I don't think it works with buttons. I haven't really tried, so I could be wrong, but I have been pushing randomly around the vent and have never seen foam.
I do however think you can see the difference from looking at the vent. I tried to get a pic of a male recently to show what I mean but it proved impossible to hold the bird, keep the feathers away from the vent and activate the camera with just two hands, so It'll have to wait till I can get someone to help me.
In the mean time, I think ivory should actually show pale pink vent feathers in males? I have no personal experience with the mutation but as far as I know only whites and tuxedos that are white in the vent area can't be sexed by the color of the vent feathers. They might still need to be around 5 weeks to show those feathers though.