A microgram of 210Po contains contains 3×1015 atoms and has got an activity of ~620,000 decays per second … so after 20 half-lives the remaining picogram of polonium would still be producing about one decay every two seconds.
The energy of the alphas emitted by polonium is distinctive - if you had an energy resolving alpha particle detector, you could count the number of ~4.9MeV alpha particles and use that to measure the polonium concentration. If you’ve got a few hours to measure integrate the counts, you can be very sensitive.
In a sense, that’s why it’s a terrible poison - if you know to look for it, you can easily detect picogram quantities, which wouldn’t be true of cyanide or other poisons.
