How can I tell if someone has seen my message?
There is no way to tell if someone has seen or read your messages.
You can tell if your message has been sent successfully by looking for the checkmark that shows up on your message on the messages page. But there is no way to track if or when someone sees your message because many people only read and respond to messages by email and there is no way to know when someone opens or reads an email.
You may also want to read the help page about not getting responses.