There's a module in the Decker examples folder that can help with this. If you look in draggable.deck in the examples folder, it explains a lot about how draggable canvases work, and it's got a module called rect that has a lot of features for checking things like overlaps built in. It might require both widgets to be "canvas" format though, I am not sure.
But basically, if you look in the example deck it should explain things. I think for what you want to do the following code added to the draggable canvas should work
on release do if rect.overlaps[bill hand] go["card56"] end end
This will need you to add the "rect" module to your deck first, so that you have the rect.overlaps function.
If I'm off base with what you're trying to do or if you have any issues let me know and we can figure it out.