Scenario:

Suppose, your Font contains 'glyphs' not available in other fonts and when the user does not have it you may want to display a link asking the user to download that font so they can view it exactly as you want.

Solution:

JFont Checker:

This is a tool for webpages to check if the font is installed. Detect installed fonts on client's machine using JFont Checker.

Basically, an element is set to use a specific font and a string is set to that element. If the font set for the element does not exist, it takes the font of the parent element. So, what they do is measure the width of the rendered string. If it matches what they expected for the desired font as opposed to the derived font, it's present.

Also Check: Javascript/CSS Font Detector