Trainees get the hang of how much speed is sufficient for an airplane to fly with time and practice. Like many have explained above, upwind the actual air speed is an addition of ground speed and wind whereas downwind it is a subtraction of the two. You judge how much speed is enough speed by looking at the airplane and judging its "energy". Some folks refer to this as "the airplane talks to you". It may sound silly but its true and it actually means that you have sufficient understanding of how the airplane flies that you can predict what is going to happen next. Atleast thats what I feel.

If there is a very strong headwind (we fly just across from the sea and it does happen), I personally prefer not to fly or to fly as much as possible upwind. I have had occassions where my trainer had flown backward wrt the ground due to the headwind. Another is to fly sufficiently high and let the airplane gain height upwind, on the downwind section you will lose height and should endup at the same height with which you started the upwing run.

Needless to say you land upwind so you fly with sufficient throttle to reach the landing strip and they throttle down and allow the airplane to settle in easily. Trainers should be most forgiving for this type of flying.

In general, I usually see what problems others are having with the wind before I take-off. If its too gusty, I either restrict my aerobatics or dont take off at all. Its personal preference

Ameyam