Great question — answering as the UphillTech team:
Just go on the BA Website . -> Then go to "Manage my Booking". There you have to enter the Booking reference and your last name, which I assume you know. If it lets you manage your booking, the itinerary is genuine.
Full answer by Daan van Hoek: https://travel.stackexchange.com/a/116133 (CC BY-SA 4.0).
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