The way I do this is to 'touch' the 'To:' field of a received message and it pops up box with the email address that the message was sent to.
So, if you had, say, two email accounts 'tim@tim.com' and 'tim@person.com' then, if you touched the 'To:' field of a received message, one of these email addresses would be displayed.
This is probably not what you want - I suspect you're looking for a way to get the iPad mail app to flag each email with the account that it was received from and I don't think there is any way to do that.
Tim
Scotland