Assumption
Suppose we are making a daily planner. One assumption is that a user can record and find today's important task without additional explanation. This is a hypothetical example, not a report of testing one of my projects.
A simple prototype
A simple flow for recording and displaying a task is enough to explore that assumption. We do not need the full feature set: the prototype should let us observe where someone starts and where they get stuck.
Observation that informs a decision
Were the labels understood? Could the saved task be found? What we observe should lead to a decision: revise the words, change the flow or abandon the assumption. Without real observation, none of these is a demonstrated successful test.