One way to do it might be to look at the Score display. Notice now where all the "sharps or flats" are showing up (if you've initially set "key of C" which is no sharps/flats).
Those "sharps and flats" form a definite pattern which is what a "key signature" on a printed score is actually showing you. (If there are "three sharps," they are always F-C-G, and so on.) Fiddle around with the key-signature setting until most if not all of the "sharp/flat symbols" go away.
And, as someone in this thread said (many years ago, now ...) "basic music theory" really isn't all that hard. There are plenty of very good videos on the subject: Toby Rush (tobyrush.com) actually did a book of cartoons. There are really only a small handful of things that you need to know to really help you out – you actually don't have to go that deep into it to come away with some really useful nuggets. You'll find yourself actually saying, "gee, that actually makes sense!"
With practice, you can actually hear what the key-signature is most likely to be. Because of "physics reasons" that we don't need to dive into, the various keys sound different, and not just at a superficial level. There's a certain discernible "pull" as the lines of melody play through: "it's not just the pitch."