On the right side of the messages you have sent, you can see green check marks. These check marks show the status of the message.
When only one check mark is shown, this means the message was sent successfully. When two check marks are shown, this means different things in a group chat and in a one-on-one conversation. In a group chat, two check marks mean that the message was received by the server. In a one-on-one conversation, the two check marks mean that the message was received by the other party of the conversation.
Apart from the check marks there is another way to check who has seen your messages. This is the “Seen by” field below your message. In the web version, you hover over the ‘Seen by’ field with your cursor. A list will appear, listing the people who have seen the message. On a mobile device you select the message and then click ‘Seen by’ in the menu that opens.