Is there a way to simulate a machine that is "clean" of the fonts required to display the page (making sure that they are downloaded) when doing development, to make sure that font-faces are being downloaded instead of assuming that they are being downloaded?
For instance if someone who isn't developing my site, and has never visited my site does not have a font on their computer, I'd like to make sure that the font is being downloaded.
Best way to make sure the person is going to get the right font for your design is to use web fonts --- With CSS3 webfonts you can now define which fonts get imported into your webpages. Make sure you use fonts with the right licenses.
Well, to answer your question - what you need to do is disable or uninstall those fonts locally in your own hard drive and see if the site still shows up properly.
If you're on a mac, your user-fonts will all be stored in /Users/yourusername/Library/Fonts
On Windows you'll have to google the location, but I believe it's a system folder on your C:\ that has windows fonts in it
Now, all you need to do is make a folder somewhere else for your 'font limbo' of disabled fonts that you don't want to be installed. Pull the font files into that 'limbo' folder and reload and test. When you're done, just place the font files BACK into the Fonts folder beside all your other installed fonts and you're good to go.
Also, if you're using Chrome (which you should) as your primary web development browser, check out it's 'Inspect Element' feature - it can show you if files are being correctly linked in or not, and allow you to tweak/update styles in CSS (without actually writing to the files) on the fly, so you can play around with the fonts there and be absolutely certain that @fontface fonts are being pulled in instead of local ones!
Hope this helps