I don't think counting pixels is reliable. Reason being, when I cycle through fonts, say in Photoshop, they always display at random sizes. I know PS isn't the web, but this goes to show that 14px in one font can be much larger than it is for another font.
I've had success taking the screenshot into Photoshop and lining it up. Of course you need to make sure your screenshot was at 100% and that you have the same font.
The best and quickest way however to see what font size a webpage is using is to view the webpage source. If it is not mentioned by
style="font-size: 14px;" somewhere in the tag, it is in the stylesheet (more than likely). Look for the link near the top of the page code that ends in .css and visit that link. Look for the
font-family declarations. That will tell you what font is used and what the size is for various elements of the webpage.