You may use the tags() function to return an array of the tags for any passage. For the current passage:
tags()
For the previous passage:
tags(previous())
An array of each moment's tags array via State.passages:
State.passages.map(function (title) { return tags(title); })
For the first two, simply use the <<print>> macro. For example:
<<print tags().join(", ")>>
<<print tags(previous()).join(", ")>>
The <Array>.join() method simply turns the array into a string with each member separated by the given delimiter.
For arrays, I'd probably suggest using an unordered list, which you can style to look like pretty much whatever you'd want. For example:
<<print "<ul><li>" + State.passages.map(function (title) { return tags(title).join(", "); }).join("</li><li>") + "</li></ul>">>