Measuring PDF margins, etc. with Preview?
Found it!

In Preview, 


open the Inspector (Command-I) 
Select the Crop and Rotate tab, whose icon looks like a small ruler. Change the units as required
Now use the select area tool and the corresponding (printed) dimensions will be shown in the Inspector.


As an alternative, you can also download Free Ruler, which displays a nice ruler on screen, though requires care in both setting the ruler DPI and ensuring that the PDF is viewed at 100% size.