I think the only way of determining the resolution of the graphics in a PDF is to use third party software like Pitstop. This software allows you to find out a lot of other info about the PDF as well.
If your concern is that the PDF file size is too large for publication on the internet, and you have access to Acrobat (full version), why not redistill the PDF if you do not have access to the original file from which the PDF was created. Redistilling is done the same way as original PDF creation.