What are some untestable questions?

Untestable questions would include the following:

If in the 1960 US Presidential elections Richard Nixon had won instead of John F. Kennedy, would Nixon also have been assassinated as Kennedy was? Would Nixon have been as successful in resolving the Cuban Missile Crisis? Would American astronauts still have landed on the moon? Would the US be better or worse off today?

This is not testable because there is no way of causing the elections to have worked out differently, or to recreate the world of 1960 in order to run an experiment of that type. We can speculate about these questions but we cannot actually test them.