Je is not intended for occasions of doubt, but rather when a preposition is needed but none of the defined prepositions are suitable. In practice, je is used infrequently, and most often in set phrases.

Time espressions (je la tria horo)

je via sano

je la barbo de Zamenhof!

plena je io (although plena de io is more common.)

io is more common.) graveda je knabo

mi kredas je Dio