How to know if someone has read my message?

I have a iPhone 5s and i mainly use iMessage to contact my friends. When I send a iMessage to some on my contacts it comes up with a 'delivered' sign at the bottom of the screen. Once they have read this message that sign turns to 'read'. I assume that it happens vise versa. However on two of my contacts once i send a message and the 'delivered' sign appears, and they have read the message, that sign doesn't turn to 'read'. It remains as delivered so i don't know if they have see it or not. How do I make it so i know when they have read my messages.

The two phones this is happening with is a iPhone 4 and iPhone 5c.

Ita just normal, not all iphones i message like it should. Try looking what company they car, if you have 2 different companies contacting it probly won't show