Some services do offer a webfont test drive, and even let you compare different operating systems and browsers. Unfortunately, most (if not all) of them are services that charge a fixed fee to use their web fonts via css.
This is the case with Typekit, where you can see the result for various outputs using Browse Samples:
I had a web project that needed some fonts a while ago, and one of the requirements was support for XP. I used Typekit's preview tool to give different fonts a score (one for each browser and each OS), and compared the overall scores of the fonts I liked in a spreadsheet. The result was quite good, I found some unexpected issues and some really polished renderings. I haven't found a similar feature in other sites like MyFonts, Fonts or Linotype, and that's one of the reasons I prefer to test in Typekit first (because of the comparing thing).
Webtype, on the other hand, also has a live preview option. What you see in the Specimens is an actual rendering of the font (using CSS'
text-rendering: optimizeLegibility;). So if you have access to browsers and OSs, you can test them by reloading the site.
Something you might find useful is Fontdeck. They have a plan that lets you download samples of fonts to use via css in your site (if you want to use them 'for real', the service costs something like $18 a year).
This article in Smashing Magazine lists other font embedding services.
Now, if you are interested in comparing fonts that come with the OS (or Google fonts), you can use an online rendering testing tool like Typetester. Or, if you have the font already, something like Web Ink or Ffffallback.