Look at the network tab in tools. If that doesn't help then try setting a break point under the script tab on $("#txtPhoneNumber").mask("(999) 9999-9999"); and see if the and then step into .mask().

Another way is to right click on the page and click on inspect element. You can go to the head tab and see if the JS files are there.

You can also open the
>I starting this project in Google Chrome. I bring up the Developer tools (F12). The first page is displayed. Then I click on the link to open
>the page where Mask() is being used. But all I see in the Developer window is that "Uncaught TypeError: Object [object Object] has no method 'mask' " But I don't know where to see if .js library is being loaded. What am I missing?
>
>>Doing web page stuff, you need to get to know the object browsers for the browser you are using. I never use IE but both Firefox with Firebug, or Chrome, my favorite, with developer tools have some excellent tools. In your case, it sounds like you are not including the js files in every page. Also, you need to use the none-mimified version of the js file when developing. Makes it much easier to step through the code in debug mode.
>>
>>>I am trying to incorporate into my project a masked input plug-in (URL: http://digitalbush.com/projects/masked-input-plugin/)
>>>
>>>When I add this .js
>>>

>>><script src="Scripts/jquery-1.7.1.js" type="text/javascript"></script>
>>><script src="Scripts/jquery.maskedinput-1.2.2.min.js" type="text/javascript"></script>
>>>

>>>to a separate .aspx file in my project, the mask() function works very well.
>>>
>>>But when I add it to a page that uses Master page, I get error:
>>>

>>>0x800a01b6 - Microsoft JScript runtime error: Object doesn't support property or method 'mask'
>>>

>>>
>>>The test code is identical in separate test page and the page that uses master page:
>>>
>>>

>>> <script type="text/javascript">
>>>        $(document).ready(function () {
>>>             $("#txtPhoneNumber").mask("(999) 9999-9999");
>>>        })
>>>    </script>
>>>

>>>
>>>The error (above) happens on the line .mask() above.
>>>
>>>How would you recommend to debug this situation? TIA