DSLRs and many modern film SLRs are made for auto-focus. There's no focussing screen when you look through the optical viewfinder, because the designers assumed that if the camera does auto-focus then the user will want to use it most of the time and have a clearer view through the viewfinder. On DSLRs that only do manual focussing, then there is usually some kind of focussing assist that makes it easy to see when focus is right. These days, mirrorless cameras in general have a focus assist feature included. On the M3 it's a highlight of the edges when the picture is in focus. You can also focus peak by zooming in on the focus point by 5x or 10x.
This makes it really much easier to use manual focus lenses than it ever was looking through an optical viewfinder.