How can I check if a Queue is empty?

Assuming you mean Queue<T> you could just use:

if (queue.Count != 0)

But why bother? Just iterate over it anyway, and if it’s empty you’ll never get into the body:

Queue<string> queue = new Queue<string>();

// It's fine to use foreach...
foreach (string x in queue)
{
    // We just won't get in here...
}