When John Doe stumbles, Beverly Crusher catches him with no effort. Seconds later when he stumbles again, Captain Picard struggles to support him.
Picard makes the verbal observation of John's cellular mutation before the image on the screen shows the cells mutating.
Although John Doe provides no memories to indicate his origin, his language is immediately picked up by the universal translator and should be a very strong clue.
After Data and the away team beam to the planet to investigate the crashed vessel, Data does a scan and says that the radiation levels are all right. The team did not beam to the planet wearing any special equipment, so the scan should have been done before they transported.
At 20:53, when Geordi and Data call the Captain to tell him about the storage capsule, Data uses a contraction, saying "We've determined that the storage capsule..."