It doesn't seem to be uniform or consistent across machine types. For the devices with the screen facing you, I usually notice that I have to ask them to let me pay with Bitcoin. There is something they have to hit on their side. When there is only a screen facing them, sometimes people let me swivel it around and tap the "other payment options" on the top left and sometimes the Bitcoin setting is there. Sometimes they have to set this up before to have it even be an option. Sometimes not. I have been to some vendors who simply have the more brick shaped device, and when going through the checkout process I noticed a Bitcoin option there when they were completely unaware. So, cause a scene and ask. Right now it seems really random to me so it doesn't hurt to try.