Under a title's entry in the catalogue it will be listed as Available or Not Available, with a link that will tell you where the item is.  

If an item is unavailable then all copies are checked out, in transit, or that the title has been ordered but not yet delivered. You can still place a hold on an item shown as Unavailable.

Alternatively, you'll see on any given item's page a box to the left of its cover image called "Find it at YPRL," which will show how many copies are in, where they are, and under what call number they appear on the shelves.