Validate a user's username and password

var webClient = new WebClient();

// replace the "admin" and "Admin's API key" with your valid user and apikey!
// http://telligent.com/community/developers/w/wiki/how-to-generate-an-api-key.aspx
var adminKey = String.Format("{0}:{1}", "Admin's API Key", "admin");
var adminKeyBase64 = Convert.ToBase64String(Encoding.UTF8.GetBytes(adminKey));

webClient.Headers.Add("Rest-User-Token", adminKeyBase64);
webClient.Headers.Add("Content-Type", "application/x-www-form-urlencoded");

var postUrl = "http://mysite.com/api.ashx/v2/users/validate.xml"; // or .json
var data = "Username=admin&Password=abc123";

webClient.UploadData(postUrl, "POST", Encoding.ASCII.GetBytes(data));

<?xml version="1.0" encoding="utf-8"?>
<Response>
  <Info />
  <Warnings />
  <Errors />
  <Valid>true</Valid>
</Response>

{
  "Valid": true,
  "Info": [],
  "Warnings": [],
  "Errors": []
}