The usual field method is to review the shot and zoom in on the critical focus area. That’s the most practical way to check sharpness on the camera.

A moderate zoom can give a quick impression, but for critical focus you want to know where your camera reaches a 1:1 pixel view. On many DSLRs, that happens somewhere around 8x to 11x playback magnification, depending on the sensor and LCD resolution. At that point, one image pixel roughly matches one screen pixel, so sharpness is easier to judge.

If you zoom beyond that, the camera may start interpolating the image for display, which can make it look softer or blockier and become less useful for evaluating focus.

So in practice:

• zoom into the exact subject detail you care about
• use enough magnification to approach the camera’s 1:1 playback view
• avoid judging sharpness at extreme zoom levels beyond that

Also keep in mind that images often look better on the camera LCD than on a larger computer screen, so playback review is helpful but not perfect.