If fsck changes nothing, it can do no damage. Normally I use -n to prohibit fsck from changing stuff, but the Solaris man page says -N works too. So it is not dangerous. How useful it is on a mounted filesystem varies. It depends on how busy the filesystem is. I would do a "sync" first (although I think that fsck does its own sync).

Your original message would bother me enough to unmount the filesystem and then do an fsck on it. When I suspect trouble, even with an unmounted filesystem, I do a -n first. Too many times I have run an interactive fsck and been stuck typing y a few hundred times. With -n, I can gauge how many and how severe the errors are. Then I can decide if my next step is fsck, fsck -y, or newfs.