They can be differentiated from other verbs because they carry a special morpheme. Another thing to note about imperative verbs is that the second aorist has a different tense stem than the present.   
- ↑ http://web.clas.ufl.edu/users/marksj/summergreek/Gk-studyguides/greek_commands.pdf
- ↑ http://www.bcbsr.com/greek/gmood.html
- ↑ http://www.mythfolklore.net/bibgreek/croy/verbs/182.htm
- ↑ Basics of Biblical Greek. Mounce, William, D.,Zondervan.GrandRapids Michigan. 2003.