There's no way to tell which messages are read. The only program that can keep track of which messages are read is the program the user is using to read the messages. There are about six popular e-mail clients at the moment and it would be to complicated to try to access them all.
Telling your mail client to delete messages from the server will solve your problem too.
Many thanks for your quick reply!
I also use webmail for my mail account, and the webmail can tell which e-mails are read. So I thought it was easier to tell. Sorry.
Many thanks again, and best regards,