Business casual for most rotations. Jeans are pretty much never acceptable. I think you are male, right? Then you have it really easy. Wear a nice polo shirt and nice pants and you are covered for pretty much anything retail or hospital or research/academic. Now, if you have something like LTC facility, jeans may be OK, but I would still suggest dressing as above and then on your first day you would see how everyone else dresses. If you have a managed care rotation or an administrative rotation, a buttoned shirt and nice trousers would be a better option.
But the easiest thing, of course, is to call and ask. It's simply a good idea to contact your preceptor about a week before the start of the rotation and ask when/where to show up, what to wear, what to bring with you, etc. For out-of-town rotations it's simply a necessity. A lot of preceptors will proactively contact students about a week or two before the rotation starts and let them know what's expected.