Assuming that it will rain when the sky is cloudy and dark is an example of a

Oxygen is a chemical element with symbol O and atomic number 8. It is a member of the from the filling of molecular orbitals formed from the atomic orbitals of the individual oxygen atoms, the filling of which results in a bond order of two. The enthalpy of formation of oxygen ("O"_2) is zero because oxygen is an element and "O"_2 is its most stable form. The standard enthalpy of formation of a ...

There can be several answers for this, actually. depends on what lesson you're on. BUT, it can range from:
Prediction
Hypothesis
Assumption
or simply guess. But scientifically, the first two would be your best bet.