I think that a good place to start would be in conversation. Instead of asking him whether or not he got your daughter something, I would like to ask if you would consider a different approach. A good way to open up the dialog is say something like,” I hope she likes what her graduation present.” This perhaps may pique his interest and ask what it is and offer an opinion. You can also say something like, “I would like for both of us to get her a graduation present from each of us but I don’t want to get her the same thing. What did/will you get her?” I think that both approaches will work for you.