Here are some steps to help determine if an email was delivered:

1. Check your sent emails folder to see if the message is in there.
2. Check your friends spam folder, in case your message ended up in there.
3. Re-send the message. Modern email clients have delivery receipts, so if the email was not received, you should be able to see that the recipient did not receive the message.
4. Contact your recipient directly to ask if they received the email.