In reading Messages From Michael, beginning at the bottom of page 75, it addresses this question. Emily asked how to identify soul levels. Michael said, "Watch for the perceptions of others. If they differ greatly from those involved, you are probably dealing with a Mature Soul. You must discriminate between fear, unrest, and craziness." She then asked if there was an easy way to see this.
He went on to say, "...Infant souls manifest fear. This can be seen in their eyes. This fear is out of proportion to the situation. The whole business of living is fearful to them. Baby Souls are guileless and this shows in their eyes. Young Souls are in a state of unrest. This often shows in erratic eye movements, the inability to hold eye contact for long. But then, the Mature Soul finds it difficult to maintain eye contact because of discomfort. Old Souls have a direct, penetrating stare absent in earlier Cycles. Wisdom is reflected there."
He went on to say, "...The social behavior of Baby Souls is normally a dead giveaway. They have none of the smoothness of the older Cycles. New situations frighten them. Change of any type is threatening. The Young Soul is usually polished and poised socially. The Mature Soul is sometimes nervous in crowds if the vibrations are bad, but they are exacting in their social relationships. The Old Soul is casual about everything. Baby Souls tend to be immaculate about person and home, have strong feelings about hygiene. They live by standard cliches, and after all, "Cleanliness is next to Godliness." The Young Soul often deeps external appearances: shove everything into the closet before the company comes. The Mature Soul goes in spurts: one day clean, the next day, not too clean. The Old Soul usually does not bother to shove anything into the closet. Who cares? The Baby Soul regularly cleans drawers, cupboards, and the tops of refrigerators."
"Let it be known that intellect is not a factor in any of these Cycles."
Hope this helps. Glad I type fast!