The question in itself is a bit vague. Talking about sharpness and to keep things simple, you have to worry about focus, vibrations and subject movement.
focus: if you are reasonably close to where you want to focus, in a portrait for example, you will obviously have zones in perfect focus (say, the eyes) and zones slightly out of focus (say the nose). What does perfect focus ( the maximum sharpness you can achieve for your system all other things being perfect) mean? Something will be in perfect focus, but maybe not what you intended. The key thing is to put your focus exactly where you want it and that's both potentially difficult and a creative decision. In landscapes, that's also a creative decision: what you want to emphasize and so on.
vibrations: typically, this would be tripod with mirror lockup in addition to perfect focus point placement. That should give you a reference point to which you can compare images taken in other situations.
There are mathematical methods that estimate the sharpness of an image, but there is no commonly used absolute "sharpness unit". The discrimination of lines on targets and how they "rise" from the background is commonly used but hardly applicable to real world images. Another method is to measure the MTF or eventutally the FWHM of point sources. Again, hardly applicable on real world images (and the MTF doesn't always correlate perfectly with the FWHM). Even in well calibrated scientific instrument one will move the focus around some definition of what the MTF should look like. MTF-50 is commonly used as it correlates well with our perception. Visually, minimizing the FWHM by adjusting focus around the optimum works well.
But, in practical photographic terms, what I do is shoot a few images as perfectly as I can (tripod, MLU, careful focus, etc...) and grow a feeling on what the best performance given a camera lens combo I can aim for is.
