All attributes in Tinderbox, System or User, are one of a number of different Data Types (String, Boolean, etc.). The type of an attribute is important in Tinderbox action code as it has some affect on how you may use the attribute and the under-the-hood assumptions Tinderbox makes when using it. How then to tell the data type for any given attribute?
User or System? If uncertain, look at the User Attributes list first, as there are usually only a few of these. If the attribute is a User one, click in the left column and look at (but don't change!) the 'type' pop-up at the top of the right side of the dialog.
If the attribute is not on the User list, it must be a System attribute. The quickest way to then find its data type is to use aTbRef's System Attribute listing, which lists all system attributes in alphabetical order.
If you don't have access to aTbRef, use the Document Inspector's System tab listing to find the attribute. Once the attribute is selected, the data type is shown on the tab.