If you use Outlook panel (and not enable EAC), you can "Log Email" manually to Salesforce for email received and sent out. Both emails received and sent will be stored both as EmailMessage record and Task record.
How to differentiate email send from Salesforce and manually logged from an email client?
You will not find any difference on the Task object, both Type and TaskSubType will populate with "Email". But there are some differences in Email Message object. Check out this query:
SELECT Id, ActivityId, FromAddress, ToAddress, FromName, IsClientManaged, MessageIdentifier, Subject, TextBody FROM EmailMessage
Row 1,4,5 - email manual log from Outlook
Row 2,3 - email sent from Salesforce
As you can see, IsClientManaged and MessageIdentifier are different.
Note: using My Email to Salesforce service (BCC) will not create Email Message record, but only Task.