If you ship reg'd with return receipt they have to sign for it, and you can check the status online (yeah, you get a #...). If they sign for it, it will show this when you go to check. Hence, they cannot receive the package without you knowing that they signed for it. You get the return receipt to prove this.
When I ship int'l, I just use normal airmail. If I can't trust the person I'm dealing with to be truthful about receiving my package, I don't deal with 'em in the first place.