ISA and SharePoint: What the port is the web application using?

You think it's an obvious question? Be hold!

In case your environment uses ISA , the url that is shown  may not reflect the actual port that your web application is using.

[What you have]:
 ISA, SharePoint Web Server. ISA resolves the url for the web application on the SharePoint.

Here is the example:
The web application url without any visible port (hence we are assuming that it's 80:

Under the inetpub folder we don't see 80 folder:

 [What you want]:
 To find out:  Where is the web.config for the SharePoint web application ? Under what port the application is running?

[What you want to do]:
You can identify the port : Central Administration > Application Management > Web Application List  

{central_administration_url}/_admin/WebApplications.aspx