sync-or-async.cs

class P
{
    static Task Main()
    {
        var task = new P().DoNothing();
        Console.WriteLine(task.IsCompleted ? "sync" : "async");
        return task;
    }
    async Task DoNothing()
    {
        const bool configureAwait = false;
        await Wait0().ConfigureAwait(configureAwait);
        await Wait0().ConfigureAwait(configureAwait);
    }
    async Task<int> Wait0() { await Task.Delay(500); return 0; }
}