I dont know what to tell you brother. If you know anyone in the Aircraft insudstry you might could sweet talk them in to x-raying the club. I dont know what all buisnesses have machines that can xray metal but i know the Aircraft industry dose. I know the military also x-rays boats, tanks air planes if you knew anyone in the service maybe you could buy them a case of beer and get them to x-ray its as a training thing but other than that i dont really know an exact way to determine if its cracked. Another way witch i just thought about was Dye Penetrant. its a liquid chemical that seaps into the any crack and if you put it under a black light it will glow if there is a crack.  you could use that to determine. if it is cracked i guess you could just order a new 6 iron to replace it. I'm sure it will be alot easyer to get ahold of some dye pen than an x-ray machine so give it a try. I've never tryed to buy dye pen my self. but i am in aviation maintenance and i know about this product.  sorry if this isnt much help.

 Just get a new 6 iron. Is there any chance they are counterfeits? I want a set of those so bad!!!!!!! They are such sweet looking irons...

Were they bent for him before you? If thats the case it could be the problem. Most clubs have a tolerance in the metal of how much they can be bent.