How can we tell what the dpi is for a pdf document? Using adobe acrobat X Pro?

Documents don't have a DPI; they aren't like a TIFF. Rather, each page on a document can have one, many or no images, each of which can have a different DPI. So it isn't a question of just getting a single number. But you can use Preflight to check the MINIMUM resolution in the file, which is probably what you want to do.