On the Dewalt you can peek through the vent holes on top with a flashlight and see how far forward the wire attached to the back of the brush is. But even that is kind of a pain - the easiest is to pop off the 4 screws holding the top cover on and that gives you full access. I think the Makita and C3D routers have brush port covers you can unscrew on the sides. I had about 150 hours of run time on my Dewalt and the brushes were wore to nothing, but YMMV.
If you have a Dewalt, here’s the full rebuild procedure. I found I had bad bearings too: