You judge by looking and comparing with every other portrait shot you have seen. Because that is a matter of taste it is impossible to give you a solid rule. However, the fact that a little softness avoids revealing every pore, hair and blemish is a good starting point. Most people will feel more flattered if they don't see those things in your shots of them.
The difficulty of doing this in camera rather than with post-processing is that you have to be careful not to draw attention away from what you want the viewer to focus on (normally the face in a portrait) because the sharpest point is elsewhere in the image. Of course, even that is negotiable. Here is a recent self portrait where I was trying to convey the sense of smell via the phrase "smell the roses" so I left myself out of the sharpest zone of focus while using post processing to bump up the detail in the flowers:
Wulf