public class ThreadUtil {
  public static int UIThreadId;

  public static void EnsureNotUIThread() {
    if ( Thread.CurrentThread.ManagedThreadId == UIThreadId ) {
      throw new InvalidOperationException("Bad thread");
    }
  }
}

Interlocked.Exchange(ref ThreadUtil.UIThreadID, Thread.CurrentThread.ManagedThreadId);

public static bool IsBackground() {
  return null == SynchronizationContext.Current;
}