How to check if login window is present with TruClient?

I am about to develop a VUGen script with TruClient (LoadRunner 12.50; Windows 8.1), and I want to be able to check if a login window is present, which is used to log in to some webpage (firefox 43.0.4). The login window looks like this:

and what I need is to fill something useful in the "if block" step:

What is there to put in besides "true" and "false"? How can I check if the login window is still there? Maybe I can define a function which returns "true" or "false" depending on some check? Can I define a TruClient function that returns a value? And how to use that value in the TruClient script then ... ?