On the View page, look at the line of buttons at the top, and ensure that the first ("List") and third ("Details") buttons are on. On the List widget you can now click on devices one by one, and in the Details widget you'll see all the properties of that device that have ever been set, their values and their type (number, string etc.).
Tip: To see properties across multiple devices, select the first device in the list, then scroll down to the last devices and shift-click to select all the devices. The Details widget will then show all the properties of all devices. This lets you see e.g. whether property "foo" is always set to "0" on all devices, or perhaps just has a few value (in which case it's a candidate to be turned into an enumerated type). Trying to select too many devices will fail, so if the list is large, just select the first 1000 devices or so.
The type of a property is automatically defined when the property is first set, and can't be changed. But you can change how it's displayed by going to Connect/Properties. For example if a property only ever has values 0, 1 & 2, which correspond to "off", "auto" and "on" modes of your device, then you can create an enumeration to automatically display these more meaningful names throughout DevicePilot.