Is there a tool that lets you simulate what a site would look like on a high-DPI/Retina display? Is it simply enough to zoom the page in to get a feel for it?
Edit:
One thing I hadn't thought of that was brought up by another commenter is that some people override the fonts used in a site's stylesheets and force only their font preference to be used (mostly for readability). This would mess with font-based icons. I can't think of a way to detect this but it's something to consider when choosing font-based icons. Maybe JavaScript betrays the actual font being used and you can detect and patch it at run-time for those users.
Sticking to glyphs that are part of standard Unicode should be fine - those should render no matter what font the browser happens to use by default.
As long as your font includes the entire Unicode character set, that is.
It would be a good idea to check that any glyph you were going to rely on for a site like HN was universally supported among common system fonts. Different platforms provide different fonts as standard, and even the likes of Arial and Verdana don't have 100% coverage.