How can you tell long a sergeant has been in the army?

If one is familiar with the Army, you can't always tell just by looking at a uniform. It will give you an idea, but to know exactly you would have to ask. On a soldiers' dress uniform, you would see a "hash mark" fpr every three (3)years of service, but these can only be added each time three years is COMPLETED. Indirectly, a soldiers rank would give a rough idea, because in order to be promoted to a certain rank, a minimum time in grade is required. Also, certain medals and ribbons are awarded for completion of required years.