Yes i'm pretty sure any hospital you're thinking of admitting yourself too would allow a visit. Please be aware though, you'll only get a clearer idea of what the place is really like once you're there - so ask lots of upfront questions and if they're honest with you that would say to me they don't have much if anything to hide.

If you intend to be a voluntary patient you're given a degree of freedom anyway. This means opportunities to leave the hospital, to walk it's grounds to have visitors etc.

Just do your research and make sure it's the kind of care that would be better received on an inpatient basis as there are lots of good treatment programs that can be had on an outpatient these days.