Schedule your visit in advance, and don't push if they're not ready for visitors.
Every new parent is different, but most need some one-on-one time with their new addition before they are ready to introduce him or her to even their best friends. Respect that timeline, whatever it may be, and simply ask if they're ready for visitors. If not, check back a few days or a week later. When the answer's yes, suggest a few times that will work for you and let the new parents decide what will work best for them.