Try in Cloudcare
When using the CommCare app builder you can use the "Try in CloudCare" button to preview your form. Click on the form settings page, and then the "Try in CloudCare" button. The preview will open in a new tab. The preview accessed by "Try in CloudCare" will always use the most recent saved version of the form (it does not matter whether or not you have made a new version of the app).
For more information see the Beginner Tutorial Part 4 - Testing in CloudCare.
If the project space has a Pro software plan or higher, CloudCare can be accessed for testing by clicking on CloudCare at the top of the page. Note that accessed CloudCare this way uses the most recent starred build.
Excel
A common way to make sure that your hidden value is working as expected is to create a "debug" label which displays the output of the hidden value. To do this for many hidden values at once, learn the technique available on the page Create a Large Debug Label with Excel.