Looking for event ID 1016 is a good start. If the exchange service account is shown as opening mailboxes when you know no other exchange administrators have been using the account, you have good evidence. If this is the case, change the password on the account and tell only the administrators you trust. When you see a failed login for that account when the suspected admin had no need to use the account, your evidence grows. (don't forget that the service account password must be changed through the Exchange Admin program - select 'Configuration' for your site, and open the properties window, then select the service account password tab). If he asks about the password not working, feign forgetfullness and give him the new password and continue monitoring for imporper mailbox access. If you see the activity you are expecting, document everything so you can have the individual terminated! Actions such as you suspect are not only an invasion of privacy, they betray the trust users place in network administrators.
Bob