It would be good to annotate them as such. Markdown does parse .onion
links correctly:
Tor Link
However, unlike normal links, it's impossible to tell where a Tor link goes. It would be good etiquette to give links like this:
DuckDuckGo (.onion)
The best solution requires SE Dev intervention, but involves adding the following to the site's stylesheet:
a[href*=".onion"]{ color:red; } # Hidden services links are styled differently
And maybe, just to be extra-sure:
a[href*=".onion.to"]{ color:orange; } # .onion.to links are styled differently again?