Your best option is probably to put the camera on a tripod and use magnified Live View to do precision manual focusing.

Some cameras support focus peaking via an electronic viewfinder or the Live View screen, which allows the camera to tell you what parts of the entire frame are most in focus. This may or may not be helpful when you are trying to isolate your subject using selective focus at a narrow aperture. Focus peaking will show what is in sharpest focus, but won't necessarily tell you if what is in sharpest focus is perfectly focused.

Your best option is probably to put the camera on a tripod and use magnified Live View to do precision manual focusing.

Some cameras support focus peaking via an electronic viewfinder or the Live View screen, which allows the camera to tell you what parts of the entire frame are most in focus. This may or may not be helpful when you are trying to isolate your subject using selective focus at a narrow aperture. Focus peaking will show what areas are in sharpest contrast, but won't necessarily tell you if what is in sharpest contrast is perfectly focused.