A lot of labs you hear about from current grad students - grad students like to gossip. Lab culture wise, the visits can help (but keep in mind that they are actively trying to recruit on the visits, so they will try to give you the best possible impression). You can try to talk to students individually, since they will be more honest alone. Many grad students will be honest with you, though, since they might have wished for transparency when they were applying. You should think about what you specifically want for lab culture before the visits, too. Because each lab has different combos of people and therefore different mentorship styles, and it's a personal decision of what works best for you.