There are many settings on the Canon 5D mark III that affect what you see and when you see it in the viewfinder. The camera does have the ability to indicate when focus is achieved over a certain point during manual focus, but only if all of the correct settings that affect the display are selected.
- Start in the menu with AF tab 3 USM lens electronic MF and set it to On.
- Be sure that AF tab 5 AF point display during focus is not set to disable. I use Selected (constant).
- Set AF tab 5 VF display illumination to On.
- Rather than selecting all of the 61 focus points, select only a few or even one using Zone AF, AF point expansion, Single-point AF, or Single-point Spot AF. If you do select all 61 points, focus confirmation will only occur for the center AF point
When I have the above settings selected and manually focus my EF 70-200mm f/2.8 L IS II attached to my 5D mark III while holding a shutter half press (or the AF-ON button), the focus display blinks red and the focus confirmation light blinks green very briefly when focus is achieved. If I move the focus ring too fast, the camera usually doesn't catch it and never flashes as I move past focus for the selected points in the viewfinder. But when I move the focus ring more slowly in the area near focus, it will very briefly blink when it detects the subject behind my selected focus point(s) is in-focus.
The Canon EOS 7D focus system is very similar in terms of set-up to the EOS 5D mark III.
For an even more detailed answer regarding what will show in the viewfinder and when it will show, please see this answer to What settings affect how the focus points are displayed for my 7D?