It often annoys me (slightly) that many (most) sites don't provide any hint to the user if a link is set to open in a new window/tab or the same one.

I'm aware that there is a class of purists who believe that the end user should be always king and they should decide whether or not a link should open in the same tab or a fresh one.

There are two problems with this.

Firstly many users are unaware of the facility to right click and select open in a new tab, and secondly there are sometimes very good reasons why a link should not reuse the same window/tab.

The first point can lead to confusion - especially when the link is to an external site (off site link) and embedded in a block of text. Suddenly the user has been taken somewhere different with only the back button to recover - but all too often the new content distracts the user and with a few more clicks he is hopelessly lost to the original page. Also on a mobile screen the open in new tab facility is even more opaque to the user; the subtle difference in timing between tap and tap and hold can confound even an experienced user. 

The prime reasons for wanting to use new tab links for a site owner include avoid the user disappearing to another site when in the middle of a piece of interaction with the first site - filling in a form perhaps - and also not to interrupt the flow of information on the page unnecessarily.  Also, if the link is to an external site, it may be more appropriate to use a new tab to explore the new content whist maintaining the position on the originating page.

I agree that using links to a new tab can be a problem for the user if there are many of them - it can lead to a proliferation of open tabs which may itself be a confusion for the user. It can also be annoying that browser right-click context menus do not have an additional open to 'open in same window' when the link is targeting a new window - despite the good intentions of the site owner the user might have her own valid reasons for wanting to stay in the same window/tab.

Whatever the merits or demerits of using target="_blank" it seems to me essential that the user is given a clear indication that it is a new tab link and somewhat different from an ordinary basic link.

My personal preference is to try and use new tabs for external links in the body of some content, but to stick to the same tab for internal links to other pages on the same site, and for external links that are not in the flow of text - for example footnotes and menus.

Providing a user Hint can be simply achieved with css that targets the target attribute of a link and inserts an appropriate icon at the end of the link.

a[target="_blank"]:after {
font-style: normal; 
font-weight:bold; 
content: "\2197";
}

where "\2197" is the css code for an appropriate utf8 glyph - in this case ↗ . If you want to use the Joomla3 IcoMoon "icon-out-2" then you need to specify IcoMoon as the font-family in the css and use "\e024" as the utf8 char..

a[target="_blank"]:after {
font-family: "IcoMoon"; 
content: "\e024";
}

Placing either of these css fragments in the main template.css file for your site will cause all new window/tab links to have the icon appended