It's actually not extremely hard.
You politely and in private ask her out for a date. If she says yes, she's into you, if not, she isn't. The answer might be something like "I'd really want to but I don't have time bla bla", that's always a "no". If there isn't a clear "yes, let's do it at this time" it's always a no.
Just don't bother with the signals, this makes it very simple. Of course only simple conceptually, you'll still need the courage to ask someone out and with like a 95% chance get rejected, but it truly is as simple as that.