1) First, follow these instructions to Install Mailtrack Add-on for Android.
2) Open your Gmail app on your phone and open an email conversation.
3) Scroll down to the very bottom of the email conversation.
4) At the very end where it says available add-ons, you will see a Mailtrack icon
5) Press the Mailtrack icon and then you should have the option to either send a tracked reply or compose a new tracked email.
6) Send a new tracked email.
7) Open the Mailtrack Add-on again and you will see if your sent email has been opened or not. Check marks should also appear in the sent mail folder of your Gmail app showing whether the email has been tracked and opened.
Please note that the check marks will only appear for tracked messages that you have sent from your computer since installing Mailtrack Add-on for Gmail, not for the previous emails.