The chance that a skillup occurs is color-coded. These colors appear in the list of craftable items, and in the tooltip of gatherable items. Exact chances for a skillup are not known. Estimates can be found by experimenting. The only certain values are for Orange (guaranteed skillup) and Grey (no skillup).
Note that when the maximum skill level is reached for the proficiency you have trained in a particular profession, you will not get any skillups. Be sure to train the next proficiency level in time.