It may seem like an absurd question to ask if your personal trainer knows how to do crunches but you'd be surprised how few trainers know how to properly do a crunch, much less teach their clients to do them!
We all know those people who tell us what they did in their workout yesterday..."I did 500 crunches....blah blah blah" and then you look at their belly and see a huge gut and wonder, "How is that possible?" The truth is that what most people think of as crunches are not real crunches at all and are not going to get them results at all.
If you want precise results, you must use precise methods. It doesn't matter if you did 10,000 crunches if they were all worthless, you're not going to get results, regardless of whether you're paying a Dallas personal trainer or not. What matters is the results you get...and the results you get are a direct consequence of the cumulative of your actions. If you want good results, you should either spend years studying the science of physical training and nutrition OR you should hire a quality Dallas personal trainer who truly knows what he/she is doing so that you can get maximum results out of your time spent in the gym.
...Then you can just let your results speak for themselves rather than having to tell everyone how many hundreds of crunches you did :)