The second step to creating a lean experiment is identifying the user's action that that can validate or invalidate your assumption. This action can take place in your application or completely outside of it.
If your assumption states that a user needs X, what action does that user demonstrate to get X? If your users won't do Y, what action do take to avoid Y? Ideally you want to find a real "currency" exchange and get the user complete the action. Something that is of value to the user.
The user action should be measurable otherwise you will not be able to identify success or failure of your experiment. I find it easiest to measure actions in occurrences.
Examples
- Assumption: Our users don't trust our application with their bank information. Action: Users enter their bank information in our application.
- Assumption: Our users need verbal support to run further. Action: Users run further.
- Assumption: Our users want the convenience of a one-click grocery buy. Action: Users click a buy now button.
- Assumption: Users want to share their music with other users. Action: Users click a share button.
Summary
Try to think of the simplest action a user could take to validate or invalidate your assumption. It doesn't always have to be in your application, but typically it is more difficult to measure an action outside of your application. It should feel obvious to you what action a user will take that gives you validation.