Yes, just counting the percent who answer the same way in both holdouts is the way to go. In a perfect world you would have 100% hits, right? But of course we don't live in a perfect world. All else being equal, higher is better and lower is worse, but we usually see about 70% hit rate for triples.
I mentioned that "all else being equal" because think about it for a minute: if all your respondents straightlined and always picked the 3rd concept, for example, then your hit rate would be 100% and you would have terrible quality data. So straightliners may will inflate your hit rate unless you changed the order of the concepts in one of your holdouts (of course, you should probably identify and eliminate straightliners anyway).