Long version: The possibilities are endless, and often not in a good way.
Easiest to track down would be an item a) currently in print, b) available for purchase, c) from a publisher who maintains a detailed website.
If an item is not currently in print, then you might be able to find a copy in the collection of a large research library. In fact, such a collection may even include copies of rental items (see below). Assuming that the score isn't rare and precious, you could then borrow a copy through Inter-Library Loan.
If an item isn't available for purchase (i.e., it's available by rental only), then you might be able to see a perusal copy. To do so, you would call the publisher and explain that you're thinking of performing the piece, and you need to see the score before you decide. Of course, to perform it, you would have to rent the materials.
If the publisher doesn't maintain a detailed website, then you could try to get more information through your sheet-music retailer. Or, if a research library has a copy, then you could learn the name of the publisher from the cataloguing information in OCLC.
If you haven't already, I would recommend that you consult with a reference librarian at Mannes and/or the New York Public Library to learn the gory details of how to hunt things down.