How can I figure out which URL is being blocked by App Transport Security?

Editor Note: @jessedc points out in the comments below the link to the official Apple documentation on how to do this: https://developer.apple.com/library/content/qa/qa1887/_index.html

Okay, I have an answer I don't like! I still very much want a better one.

In my application:didFinishLaunchingWithOptions: method I added the line

setenv("CFNETWORK_DIAGNOSTICS", "3", 1);

When I ran the app then, in the log I can find an entry that looks like this:

2015-07-02 15:27:56.152 MyApp[45041:9125662] CFNetwork diagnostics log filecreated at: /Users/micah.hainline/Library/Developer/CoreSimulator/Devices/11BCA581-5F5F-494D-932A-2ECFCA33EA93/data/Containers/Data/Application/9ACC6941-8039-4B86-B5E8-A6C66E2AD520/Library/Logs/CrashReporter/CFNetwork_com.myapp.MyApp_45041.nwlrb.log

When I open that file I have a huge number of log entries about everything that's happened on the network. I search for kCFErrorDomainCFNetwork in that file and get logs for failed network requests. I can use that to see what URLs the system was trying to hit, and then can add that URL to the exceptions for App Transport Security.

Accepted answer is correct but If you're doing this on Xamarin iOS, you will need to use;

Environment.SetEnvironmentVariable("CFNETWORK_DIAGNOSTICS", "3", EnvironmentVariableTarget.Process);

Also, if you're looking for the log file use something like Simulator manager to find the correct location easier (https://github.com/tue-savvy/SimulatorManager).