How do I tell other users/contacts my iMessage is turned off?
As far as I know you can only do that manually. 

There are few differences between a Text Message and an iMessage. 


The most obvious an iMessage has a blue colored background and a regular Text Message has it's background green

The name of the contact/number you are sending to will turn blue or green respectively for iMessage and for a regular Text Message

After resolving the phone number of the receiver(described above), the text box will automatically say what kind of message will be send and the "Send" button will be colored blue or green respectively for iMessage and for a regular Text Message