From what I can tell the only UI that can do CSS autocomplete queries for pseudo selectors is the StyleEditor.
From the StyleEditor, if you start typing eg
div:, you will see
div::marker in the suggestions, but as soon as you type the second
:, the suggestion disappears.
It looks like this autocomplete only supports pseudos with a single
: at the moment. It would be nice to file a follow up to fix this.