The following will tell you whether or not all of the text is selected within a text input in all major browsers.

Example: http://www.jsfiddle.net/9Q23E/

Code:

function isTextSelected(input) {
    if (typeof input.selectionStart == "number") {
        return input.selectionStart == 0 && input.selectionEnd == input.value.length;
    } else if (typeof document.selection != "undefined") {
        input.focus();
        return document.selection.createRange().text == input.value;
    }
}

2017 Specific Answer - Faced the same issue recently.

We were allowing users to enter only 3 digits at a time. When the user tried to enter the fourth character we returned false.

This became an issue when the user had a selection and was trying to overwrite the values.

Taking a hint from Tim's answer. I understood that I wanted to see if the selection value was same as the input's value.

In modern browsers I achieved it by doing:

document.getSelection().toString() === input.value // edited

Hope this helps someone.

For anyone who needs the code to get at the selected text within a textbox, here's an enhanced version:

http://jsfiddle.net/9Q23E/527/

function getSelection(textbox) 
{
   var selectedText = null;
   var activeElement = document.activeElement;

   // all browsers (including IE9 and up), except IE before version 9 
   if(window.getSelection && activeElement && 
      (activeElement.tagName.toLowerCase() == "textarea" || (activeElement.tagName.toLowerCase() == "input" && activeElement.type.toLowerCase() == "text")) &&
      activeElement === textbox) 
   {
      var startIndex = textbox.selectionStart;
      var endIndex = textbox.selectionEnd;

      if(endIndex - startIndex > 0)
      {
          var text = textbox.value;
          selectedText = text.substring(textbox.selectionStart, textbox.selectionEnd);
      }
   }
   else if (document.selection && document.selection.type == "Text" &&  document.selection.createRange) // All Internet Explorer
   {       
       var range = document.selection.createRange();
       selectedText = range.text;
   }    

    return selectedText;
}