Check for a read receipt or “Seen” status in the messaging app
Look for a delivered status followed by a read status
See if the person replies soon after receiving the message
Notice if they react to the message without replying
Check whether the app shows a timestamp for when the message was opened
Verify whether read receipts are enabled for both you and the other person
Remember that some apps and devices do not support read receipts
Consider that notifications may let someone preview a message without opening it
Look for typing indicators after the message is sent
Check if the message appears in a chat thread as read on both devices