Deductions

Even spare travelers untrained in either astrophysics or planetology have enough general knowledge to make deductions about new planetary systems and environments.

Because the planet Nike's sun flares a lot, Roan Tom's senior wife, Dagny, deduces that:

anomalous chemical composition has so far prevented this sun from stabilizing at its proper position on the main sequence;

this implies that the sun, and therefore also its planetary system, is young;

primitive life forms also imply a low age;

such a young planetary system will have condensed from an interstellar medium metallically enriched by the novae of an earlier stellar generation;

therefore, local fruit and water may contain lethal quantities of, e.g., arsenic, selenium or radioactive elements.

Thus, Dagny deduces poisonous local food from solar flares. However, she remains puzzled as to how such a small planet has retained a terrestroid atmosphere. Maybe the system is so young that there has not been enough time for the atmosphere to be lost? But she knows that her knowledge of cosmology and cosmogony is derived from an oral traditional that has become a modern myth and that she has not been trained to apply scientific method so she does not speculate further.

Then, at the same time, other factors have to be considered:

Dagny and Yasmin must take risks to survive;
there is a human population on Nike;
such a population may have adapted to a metal-rich diet;
however, Terrestrial plants and animals have become dominant after a mere thousand years;
therefore, the adaptation cannot have been large;
it follows that Nike should be biochemically safe at least in the short term.

Dagny and Yasmin eat the apples and drink the water.