What actually separates a BMW specialist from a garage that just says they are?

I've been burned once already by a place that advertised itself as European car experts and then spent three days trying to figure out a simple sensor fault, so now I'm a lot more careful about who touches my car. I'm looking for BMW Garage maintenance specialists this time, and I want to actually understand what that means before I commit. Is it just about having the right diagnostic software, or should I be asking about things like whether the technicians trained with the brand, whether they use original parts as standard, and whether they can handle the coding side of things without needing to farm it out elsewhere. I've spoken to two places so far and both sounded convincing on the phone, which honestly makes it harder rather than easier. For those of you who've found a good independent, what questions actually helped you separate the real specialists from the ones who just have a nice website and a lot of confidence? I'd rather ask slightly too many questions upfront than find out the hard way again.