How do you know if symptoms are from low testosterone?

This is something I’ve been debating lately—how do you actually know if you’re dealing with low testosterone? Fatigue, low energy, and mood swings can be caused by so many other things like stress or poor sleep. So what are the specific signs that should make someone think, “Okay, this might be a testosterone issue”?

Do changes in the gym—like struggling to build muscle or losing strength—show up early? Or is it more subtle at first, like focus and motivation slipping? And when it comes to sexual health, do problems there always connect to testosterone, or could they be from other factors?

I stumbled on this discussion: https://chadbites.com/blogs/research/top-signs-of-low-t-what-you-need-to-know and it listed several warning signs, but I still wonder how reliable those really are. Can symptoms alone be trusted, or is bloodwork the only true way to know? For those who’ve dealt with this, what was the first red flag you noticed?