1. Your new emails are marked as Read even if you’ve not read them.
2. Your emails are moved to Trash or even permanently deleted without your notice.
3. Your emails are being forwarded to a third party email address (check your settings->forwarding).
4. Your secondary email address is changed.
If you come across any of the above activities on your email account, then it is a clear indication that your email account is hacked.
Steps to be carried out to stop unauthorized activity on your email account
If you feel/suspect that your account is hacked then you must immediately take the actions mentioned below
1. Change your Password
2. Change your security question.
2. Remove any third party email address (if any) to which your account is set to forward emails.
3. Make sure that you can access the email account of your secondary email address.
4. Also change you secondary email password and security question.
This ensures that your account is safe from future attacks. But I strongly recommend that you read the following post to protect your email account from being hacked