Open the WhatsApp group chat
Tap and hold the message you sent
Tap Info
Check the Read by section to see who read it
Check the Delivered to section to see who received it
If the message is a group message, read receipts show only for members who have enabled them
If a member has turned off read receipts, you may not see that they read the message
If someone has not opened the chat, they will appear under Delivered to instead of Read by