Go to form builder. Open menu "Actions". Select option "Run" to open proper popup:

Popup "Run" allows to pass some data to the form (available from version 5.0):

• the record Id (to display form for a specified record)
• the process instance (to display form like it will be shown on specified process instance)
• custom parameters (if your form needs them, you can pass them manually. e.g. for testing purposes)

From the version 8.7 it also allows to select a mode (Edit or View)

In case you do not need this, just click "Run" button to open the form preview in another browser tab.

Check "Pass Record" checkbox - the record selector will be shown. Type ID into field or click settings button to open list of Records names (see picture below):

Filter by typing a part of record name, if necessary, and select Record Name by clicking a proper row. Click "Apply" to pass Record ID:

Check "Pass Process Instance" checkbox. Type Process instance ID or click settings button to open a list of processes:

Filter and select process version. Click "Apply" to pass process instance ID. Form renderer will get this ID in "workflow" parameter.

Check "Pass Custom Parameters" checkbox. Proper sub-form will be shown. Click "Add" button and set parameters. Type name and value of parameters to be passed to the form.

Finally click "Run" button to preview the form.