Agree with definitely ask the parents. Some things are portable and they may want to bring their own.
You might ask if it would be helpful to them for you to purchase a couple of boxes of diapers or something to keep at your home, just in case they run out of diapers while visiting. You can explain that it's not that you think they're incapable of packing enough, you just thought it might be helpful to have a few extra on hand in case of a major blowout or something.
If you're wanting to buy them a baby gift, I would suggest offering to give them money to purchase a large item, such as a crib. This way they can select what they like. Another alternative would be to ask if it would be alright to make something for the baby, if you are crafty. I sew, so I like to make stuffed toys out of socks (I use dental floss instead of sewing thread, so the toy won't come apart), an embroidered birth record, or an embroidered quilt. If it's a quilt or birth record I like to let the parents know and then ask if they have favourite colours or a theme for the nursery, or conversely if there's anything they really DON'T want. (Although the pink embroidered castle quilt is adorable for a girl, some parents don't care for the "pink princess" idea.) That way I can select something they will like.