When you message someone, he will receive a red notification in the top bar on the message icon near to his profile image. The number inside the circle indicated the number of your unread messages.
Also if he/she turned on email notifications he/she will receive an email confirmation.
If he/she has Baaz mobile app he will receive a push notification and will see an update for the discussions in the menu. Also he will be able to see how much messages he did not read for each conversation in a blue circle next to the conversation.