The short of it is....you can't. Unless you go to a store and they allow you to take the scope outside when it's getting dark. Then again, if you really want to evaluate a scope's low light performance, you have to compare it, which means that you will have to take at least 2 scopes out there. Good luck finding a store that will allow you to do that. I guess that smaller mom and pop shops might help you out like that, but even then it seems kinda hard for the most part.
Truthfully, one of your best resources are people like us on these forums. Many members, including myself, own several brands of scopes and thus have the opportunity to do our own evaluations and post them to other members. Yes, 2 people can see things differently and thus such tests can be subjective, but try and make a decision based on these evaluations, subjective as they may be, because they really can be helpful, and, for the most part, unbiased.
Zachary