I heard that Topjet exchange. I was somewhat surprised that the fighter apparantly did not know the 757's callsign, so it took a few tries before the Topjet crew realized that in fact they were that "757 aircraft on flight level XXX, position YYYY". I think the situation was professionally dealt with by both parties - goes to show the importance of the guard frequency.
"You're on guard" may sound clear and consise, but I have heard several instances where the guy transmitting on the wrong frequency in fact took it for "go ahead", and started an even longer erroneous transmission with ETA's and fuel and wheelchair passengers and what not.
People don't realize that "you're on guard" is actually aviation slang used mainly by native english speakers (and, for some strange reason, by grumpy Germans). Despite being a longer transmission, "<Callsign>, you are transmitting on 121 decimal 5" might be a lot more effective, if the need to play guard-police really gets the better of you.