Thanks everyone for your replies.
My case is more proving what it's not, rather than what it is. The specification in question only allows GTAW for repair. It is suspected that in a few cases, SMAW was used, and in others GMAW. I don't think there is a none destructive method, but we do have some cut samples to play with. I am thinking of wet chemical (optical emission spectroscopy) as the spark emission is not as accurate. SEM for imaging, as well as optical microscope, and as you suggested cross sectionals, longitudinal and transverse. I don't have to prove what it is, only what it is not. Js55, I didn't think of the non metallic inclusions, but thats a good point.
If anyone has any ideas of how to improve on this methodology please speak up.
Thanks again for everyone who has provided input.
Regards,
Gerald