[ReaLNero]

There were unusual elements characteristic of the decay chain following a fission.

After a U-235 atom undergoes fission, one of the outcomes is it releases Barium and Krypton (and some neutrons), which then eventually decay to stable/semi-stable elements. If one of those stable elements is common in the deposit but otherwise rare naturally, it would point to a nuclear reaction having occurred.

Also note that the U-235 decay chain generally looks different from the decay chain following a fission reaction of U-235.