In general you do not need to worry about various Stacks being responsive. The key, instead, is whether the theme you are using is responsive. There might be a very small number of stacks out there that are non-responsive but they'd be hard to find/buy at this point!
If you are using Foundation then you are in good shape (as Scott mentioned). But there are also plenty of other themes these days that are responsive. I'd guess well over 90% of them are.
But as Scott also mentioned the specifics of "how" that responsiveness is implemented and the implications for designing a website are definitely different for some themes (Foundation, Foundry, and some others) relative to the approach most themes take.