what does the weight sticker on the door jam of your celica say? the weight of a celica gt with sun roof is slightly heaverier, same for gts. does the sun roof look like other celica sunroofs? what about the center controls, are they stock? run a carfax, for any "roll over" incidents or other types of accidents that may be related to why you think someone would replace the roof as you belive.
one last thing to check, your VIN is ingraved all over the vehicle engine, windows, trunk. can you find that same VIN on your sun roof?