Simply ask DIL if she appreciates your gift.  If she asks, why you ask this, let her know that you didn't hear a response about the gift so are wondering how she feels about it.  This gives her a chance to be honest - maybe she would prefer that you donate to a charity on her behalf or simply not give a gift.  Or maybe she doesn't realize that a response is welcomed.  Possibly she's telling your son to be sure to tell your parents how great the gift is, and that message isn't getting through to you.   
Clear the air, there is nothing wrong in an honest exchange - seems like you will always wonder..