How can I find out when the message that I have sent from gmail has been read?

you can't find that, gmail doesn't have such feature, at least that's what i know, but i truly doubt that they have one.. not yet but probably in the nearest future they'll implement that and probably it'll be the first mail system from dozens of free e-mails that doesn't have it.. instead you can get the gmail notifier whether to see you recevied e-mails on your inbox or not.. it's just like being alerted :)

in your email options you should have a "read receipt" option to check off. When the email has been read, you will get an email back telling you