How to identify if a webpage is being loaded inside an iframe or directly into the browser window?

Browsers can block access to window.top due to same origin policy. IE bugs also take place. Here's the working code:

function inIframe () {
    try {
        return window.self !== window.top;
    } catch (e) {
        return true;
    }
}

top and self are both window objects (along with parent), so you're seeing if your window is the top window.

When in an iframe on the same origin as the parent, the window.frameElement method returns the element (e.g. iframe or object) in which the window is embedded. Otherwise, if browsing in a top-level context, or if the parent and the child frame have different origins, it will evaluate to null.

window.frameElement
  ? 'embedded in iframe or object'
  : 'not embedded or cross-origin'

This is an HTML Standard with basic support in all modern browsers.

RoBorg is correct, but I wanted to add a side note.

In IE7/IE8 when Microsoft added Tabs to their browser they broke one thing that will cause havoc with your JS if you are not careful.

Imagine this page layout:

MainPage.html
  IframedPage1.html   (named "foo")
  IframedPage2.html   (named "bar")
    IframedPage3.html (named "baz")

Now in frame "baz" you click a link (no target, loads in the "baz" frame) it works fine.

If the page that gets loaded, lets call it special.html, uses JS to check if "it" has a parent frame named "bar" it will return true (expected).

Now lets say that the special.html page when it loads, checks the parent frame (for existence and its name, and if it is "bar" it reloads itself in the bar frame. e.g.

if(window.parent && window.parent.name == 'bar'){
  window.parent.location = self.location;
}

So far so good. Now comes the bug.

Lets say instead of clicking on the original link like normal, and loading the special.html page in the "baz" frame, you middle-clicked it or chose to open it in a new Tab.

When that new tab loads (with no parent frames at all!) IE will enter an endless loop of page loading! because IE "copies over" the frame structure in JavaScript such that the new tab DOES have a parent, and that parent HAS the name "bar".

The good news, is that checking:

if(self == top){
  //this returns true!
}

in that new tab does return true, and thus you can test for this odd condition.