Maybe, maybe not. If the people who control the mail server don't want you to know and they have access to the server then it's possible for them to access the email without the account owners knowledge.
If they have a copy+forward it might be possible to find out if you send several emails with BIG email attachments to the account. You'll find out if you start getting bounces back from a 3rd and unexpected account. Try sending several large email attachments, clear out the account you are testing before it goes over quota (runs out of space), and send them a few more times. Repeat a couple times.
If the account has a login history for webmail, pop3, and imap connections then it's also possible you could see multiple IP addresses connecting to the account.