How do I capture the SAML response right before getting directed to the callback URL after login?

To illustrate:

1. User clicks "SSO login" in your own app
2. You direct user to the SAML app's login page (e.g. Okta's login page) by calling IdP_SSO_URL to generate the URL (of course you pass in the callback URL e.g. "/App1/Entry.aspx")
3. User successfully logs in to the SAML app (e.g. Okta)
4. User gets directed to "/App1/Entry.aspx"

I am sure there's a SAML response or a more informative URL right before user gets directed to "/App1/Entry.aspx". I believe this is where you can parse the response/URL for the user's permissions/details/etc. How do I capture the SAML response/informative URL right before getting directed to "/App1/Entry.aspx"?

Thanks!