Mirages could have had any color interior. Also the Mirage package could have been ordered for just about any 1977 Monza 2+2 regardless of engine choice.
They were all painted white. The interiors could have been white, black, red , etc. I have had 2 both had red interiors although i would prefer white with red accents (dash, carpets etc).
There is no definitive way to verify the authenticity of a Mirage (well except that it must be a 1977 Monza 2+2). One possible tell tale sign is blue and red over spray in the cowl area, that part of the Mirages was painted. However if a car has been repainted those might be gone.
You can check the Mirage Registry - possibly your car was listed there by a previous owner - again not positive proof , but circumstantial evidence at least.
http://www.monzamirage.com/