I use an electric focuser because it adjust focus without introducing vibrations.
To fully determine the exact point where the image is focused I use a Hartmann mask with four triangles, because diffraction peaks generated greatly facilitate the determination of exact focus. You can make a template for this kind of mask here.
Twin diffraction peaks merge when they reach the focus.
This mask is a marvel when it comes to focus the camera, as it not only is more luminous than Hartmann mask and clearly shows when you reach exact focus, but also you can clearly see which side of focus you are at all time.
You can make a custom template for this type of mask here . This mask is much better than a Hartmann mask but it is harder to make as it has many openings to be cut accurately. You can use the software Bahtinov Grabber (freeware) to help excact focus detection.