Are we talking about an aircraft from a manufacturer with all that goes with it, or a kit on a permit?
If you think it came from a factory, check the manufacturer's plate with the serial number. Then cross-check that against G-INFO. It's possible for someone to fraudulently make a plate, but they'd have to make up a serial number. You can check against the manufacturer's records. It's a bit like the VN on a car - hard to completely defraud in the long run.
There have been aircraft rebuilt using little more than the manufacturer's plate.
If it's a kit or on a permit, check with the LAA.
TOO