You can get a decent enough hygrometer to measure humidity for like $10 to $15. While not at all a necessity, getting your work environment to be as stable of a humidity level as possible will make instrument building easier. Just mentioning it because you mentioned a window ac unit which can swing humidity when cycling.
But once again, it’s not at all a necessity to keep the humidity stable if it would be difficult to achieve in your space. You can still get really great results with humidity swings, and a whole lot of builders do, you’ll just need to pay more attention to your wood, and especially for wood species that move a lot, plan with movement in mind, especially in terms of when you’re glueing things up.