I've been searching for clear guidelines about how(or even if) you should present external links in an html document in order to conform with any accessibility/usability guidelines. But I can't really find anything.

We have a client that insists that any external links(including menus/navs) should have an icon indicating that it is external.

My question is - is this necessary? What are the minimum requirements for distinguishing external links? Does there need to be a visual clue or is it possible to just indicate in the link title, or name, that if clicked it will go to another website?

Any clarification on this would be greatly appreciated as I'm struggling to advise what we should do.