How do I check my sp_Blitz/sp_WhoIsActive versions across multiple servers/databases?
For sp_Blitz, there's an enhancement request filed at Github to add outputs for version, and if you'd like to influence how the work is done (or contribute code), you're welcome to leave comments over there.

For sp_WhoIsActive, the author Adam Machanic is considering adding an output to the Messages tab each time it runs.

Until then, Oreo's suggestion above of checking the code is the best one. We won't break that intentionally - we haven't changed those strings in years, and don't intend to start, heh.