Detect redirect in iFrame

I may be asking the wrong question here so I will provide a small amount of detail about what I am trying to accomplish.

I use a third party web app to track support tickets. They provide the code for a form that my users fill out and it submits to their domain. I want to use this form on two different domains, but unfortunately the third party uses a single, hard coded redirect value. This causes one of my sites to switch domains after submission.

I was thinking I could embed their form in an iFrame, detect the redirection on successful form submission and instead redirect to one of my own pages.

Pseudocode:

iframe.detectRedirection
if (redirect == 'xyz.html') {  
     //do my own redirection in main window.
}

So back to my original question - how can I detect the redirection in the iframe?