I just dip a toothpick in to get a little taste first, or a drop on the finger if your brave.
It may give an you an idea of the strength & profile.
Hard to know if it's a main flavor or an add on, as the latter can be tough to do as a stand alone.
But 5% is usually a good starting point unless it's a really strong flavor.
It could also be a pre-mixed house flavoring meant to be used all alone...