I am considering writing an alternate history story in which 1950s nuclear testing attracts the attention of benevolent extraterrestrials who do not want to see us destroy ourselves. However a misunderstanding ensues between the ambassadorial parties sent by the superpowers to meet the aliens; the whole affair goes south in a big way.
I looked up the various tests done by both superpowers in this era and it seems that the Castle Bravo test represented a major upward spike in blast force and radioactive dispersion. How would an alien probe stationed somewhere in our Solar System (you can decide where) be able to detect this event? Would smaller blasts also be detectable? Let's assume the aliens have a means of FTL which permits their probe to communicate to their homeworld "Hey guys. The monkeys from Sol III are testing megaton-level nukes now. Maybe we should have The Talk?"
Just to clarify, I am not asking about a global nuclear holocaust scenario. I am specifically referring to the "controlled" atmospheric tests conducted in the early years of the Cold War.
Edit a few hours after post: Thank you for the answers, everyone. Keep them coming, I'm all ears!