When you travel to the astral plane, or when you start allowing communication to come to you from the other planes, the messages will come to you first in ways you are used to receiving spiritual, subtle messages. This may be with dreams, or in meditations, or with music and songs, for example. It may even come in thoughts or via telepathy. In time, and with experience, you will be learn how to distinguish these types of subtle messages from other types of communication.
The ability to receive communication from the astral plane is one of the benefits of cultivating spiritual, subtle energy.