You have two ways to preview the reactions you create, before actually launching them on the site:
Preview within the Studio -
When creating your reaction, and at any point after, you can play the reaction within the Studio to see what it looks like.
While creating or editing the reaction, simply click on one of the "Play" buttons:
View the Page as Visitor -
You also have the option to visit a simulator, which appears like the page you are testing.
On this page you will be able to trigger the reaction, just like a user would, by behaving in the "right way" (clicking on the element, showing exit intent, etc.).
To open the simulator, click on the "Eye" Icon at the top right: