Before the launch of a product and thorough measurement of user behavior, everything is a hypothesis.
You always have to create hypothesis and test it. Otherwise, you are just guessing. Create a simple assumption like: more people will click this red button. Indicate the metrics, like percent of clicks.
Or use a more complicated format like:
We believe that [ doing this / building this feature / creating this experience], for [these people/personas], will achieve [this outcome]. We will know this to be true when we see [this feedback / quantitative measure / qualitative insight ].
Then test the hypothesis. Remember, you can be affected by confirmation bias, which means you may try to prove yourself right, instead of just testing the hypothesis. Be careful and try to see where you are wrong.
We never know what is successful if we can’t measure it. Any opinion without data has no practical value.