First of all, if the welder suspected of using a 7018 to weld 304 stainless steel cannot tell the difference in a carbon steel rod and a stainless steel rod, he does not need to be trying to weld stainless in the first place. One real quick way to tell is to use a "pencil magnent" and check the magnetism of the pass suspected of being done with a 7018. Even if, by some chance, the welder made any dilution with the 304 base metal, the weld bead will exhibit much more magnetism than the 304 base metal. If you cannot get to the first pass, either from the inside or outside, you might have to do a chemical analysis to be sure. Either way, if you even think it was done with a 7018, it would probably be a good idea to remove that weld and make sure you are certain to use a stainless steel filler metal (usually a 308 or 308L) to do it right.