How to Know if a Message Was Read on Facebook Messenger
Look for your friend's picture (on Mobile) or the word "Seen" (on Facebook.com) at the bottom right of your message.
Open the Facebook Messenger app. If prompted, enter your username and password and tap Log in. Tap Home. This is in the menu bar at the bottom (iPhone) or top (Android) of the screen. Tap the conversation you want to check.  Look for your friend's picture in the message window. This will appear at the bottom and to the right of one of the messages. The small picture demarcates the last one read by your friend. Any messages following the one with the picture have not yet been read. A small blue check mark instead of the small picture indicates that the message was successfully sent, but not necessarily viewed. Navigate to Facebook in your browser. If prompted, enter your username and password and click Log in. Click Messenger. This is in the left sidebar. Click the conversation you want to check.  Look for the text "Seen". This will appear at the bottom and to the right of one of the messages along with a checkmark. The text signifies the last message read by the recipient. Any messages following the one with the picture have not yet been read. 
Open the Facebook Messenger app. Tap Home. Tap the conversation you want to check. Look for your friend's picture in the message window. Navigate to Facebook in your browser. Click Messenger. Click the conversation you want to check. Look for the text "Seen". 