I have received more email about confusion over the active app in the few months since Big Sur was released. Big Sur is brighter, and figuring out which window is active is more difficult than it ever has been before.

The differences are subtle:

Traffic Lights
The close / minimize / maximize buttons are lit up in the active window and gray in all inactive windows. This is the easiest way to tell.
Drop Shadow
There is a subtle drop shadow behind the active window. There is no drop shadow on inactive windows. Depending on the background, this can be impossible to see.
The App Toolbar
The apps toolbar is just slightly darker in the inactive window. Unless you have the apps right next to each other, this is difficult to notice.

The only reliable way I can tell which app is active is the traffic lights. If you find this annoying, I would suggest HazeOver (Developer) (Setapp). This simple utility darkens everything on your screen except the active app and solves this problem entirely. The app has been around a long time. It got a nice Big Sur update and fully supports Apple silicon.