It's been a few years since I've used Adobe to create forms. The last one I used was Adobe Live cycle and there was a preview tab that I could use to preview my form as I was working on it to be sure the fields were working as I wanted. How would I go about doing in this new Forms Central?
I'm guessing that you are creating PDF forms with FormsCentral for Acrobat. What you need to do to "preview" your form is to actually export to PDF then open the PDF in Acrobat or Reader.
If you are not signed-in into the service or if the form is stored locally you will be able to export via the "File > Export as PDF Form..." from the Design Tab
If you are signed-in into the service and the form is stored online you can also get the PDF from the Distribute Tab, select the PDF Form Tab and click on the Download Button.
Hope this helps