Before Skill Cards can be used, their Use Condition must be met. They are inferred or specified in their descriptions, and cannot be used if they are not met.
The constant condition is that you may only use Skill Cards on your turn and if you have not already used another before your dice roll, even if this is not specified on the card. Otherwise, it cannot be used.
Some Skill Cards have no special Use Conditions. However, this does not mean they will always activate. For example, when using Smack!, the card can be used but the player needs to roll in the specified range to trigger the effect.
Skill Cards that let you select a city require a city to be present before it can be used.
Some Skill Cards may be used even if the situation is not beneficial, for example using Symbol of Wealth even when you own none of the existing Lv. 4 cities.