The easiest way to determine if the person is under 19 is to look at their birthdate. This requires some calculation, but there is nothing special to indicate that the holder of the driver's license is under the age of 19.
I'm sure it depends on where you are from, but some licenses say something like "Turns 18 on _________" and "Turns 21 on ___________" with a date to make it even easier to determine what the person can and can't do legally.