Phone battery drains fast

My phone battery has been dying much faster than it used to. It's an Android, and it used to last a full day, now it's down to maybe half by mid afternoon even when I barely touch it. I looked in the battery settings and there's a list of apps with percentages, but nothing looks crazy to me. The biggest one is just the screen, and I don't know what to make of that.

I've only restarted it and turned the brightness down a bit, which didn't change much. Is there a way to tell which app is really the problem without buying anything or installing some random tool? And how do I know if it's an app misbehaving versus the battery just getting old? What would you check first?