You can preview reactions you've created in several ways.
Preview in the Web Personalization Studio
You can preview Reactions from the Studio by selecting the Reactions option from the left navigation.
Scroll to the desired Reaction and click Edit.
Next, click the reaction type or play icon (either one) to preview your reaction.
Preview in the Web Personalization "Mirror" Site
This option opens a new browser tab and allows you to see the site as a visitor. You can duplicate the triggered behaviors and view Reactions as your customers would. In the Studio, click on the eye icon to begin.
When using this option, you will need to turn "On" your Reactions in order to preview.
Preview on your website with a test URL
If you are not prepared to turn on the Reaction, there is another option to preview by creating a test URL parameter in your Persona.
Click Create a New Persona, scroll down to URL Parameters and click the gear icon to create your own parameter. Add a test URL parameter like the following:
Click Add, then save changes to return to the persona. Now click on "URL Parameters" and add the new value you created above.
Save the new persona.
This will give you the control to turn on a Reaction with this persona and it will not be available to anyone else unless they have your unique URL parameter.
To append to the page URL you are testing on add ?key=value (based on the above example it looks like this: http://website.com?stage=test).
Does everything Preview as expected and now you are ready to launch to the public? Remove the Persona from your reaction and you will be good to go!