Pass Credentials on a SharePoint site and get a Response

Question

I'm trying to pass credentials against a SharePoint site (It uses Windows Authentication). This is the code I'm using to do so,

SPServices.Lists sourceListService = new SPServices.Lists();
        System.Net.NetworkCredential networkCredentials = new System.Net.NetworkCredential("username", "password");
        sourceListService.Credentials = networkCredentials;

I would like to know how I can receive a response from this so that I can then use an If statement to check if the Username and Password entered in the Windows Form was accepted by the SharePoint site.

For example,

if (SPresponse = allowed)
        {
            Do something 
        }

Answer 1

I would send a POST to ../_api/contextinfo

If the credentials are valid, then the call will return a FormDigest, which you would likely need anyway.