When using the CommCare app builder you can use the App Preview bar. Click on any area of the project, and then find and click the blue App Preview bar as illustrated on the image below. The preview will open on the right side of the screen in the same tab and will provide a preview of the entire project as if you are using a mobile device. To preview a specific form you are working on, go through the app workflow that leads you to the form by clicking the buttons on the preview screen. The App Preview tool will always use the most recent saved version of the project (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.
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.