Hi Kathleen,
I feel for you! I had a similar (but temporary) experience in a hotel once. I have to say that I disagree with the other ladies about calling DSS, at least without talking to the parents first. Imagine the mother's position there, if some random stranger dared to butt in anonymously - not even bold enough to confront the mother herself, smearing her motherhood by dragging DSS into her life!
I do think, for your own peace of mind, that you should strike up a conversation, either when you are hearing the baby cry (maybe go offer to watch the child for a while to give the dad a break??) or offer a quick "playdate" with your child to distract the child from missing her mother (where you could also watch the dad interact with his baby...like someone said...the dad COULD be holding her, etc to no avail). Or you could follow a previous suggestion of talking to the mom and letting the conversation wander to dads and babies.
I wish you the best of luck, and you'll always wonder if you don't do ANYthing, but at the same time, please don't drag a family through DSS if you haven't gotten more concrete proof of what's going on.