Check the rear of the barrels. If there's a serial number stamped on it, it's likely been replated. Not all replaters do this though, so no mark doesn't mean much!

If it's just been rebuilt, there'll be witness marks on the head nuts, and probably bits of stray goo round the exhausts. There shouldn't be any spooge at the headers if they've cleaned it up. The powervalves should move freely if it's been de-coked too.
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball