There are many tell tale signs for 49 shell, I can think of a few off the top of my head.
The chassis number should start BFAT (make sure log book and car match) there should also be another plate on the right of the slam panel, this will be an avo plate and will contain the details of the original spec of the car. As for the shell itself look for strengthening plates on the inner wings, strut tops double plated, underneath the Boot should have studs showing for the fitment a stone guard (if it has the stone guard even better but it's rare), anti trap bars (radius arms) fitted from the axle facing forward fitting onto the chassis. That's all I can thinks of at this time in the morning! Hope this helps