First step would be to identify them by pulling all elements of the desired category in the view. Revit 2022 and on should have “All Elements of Category in View” as an out of the box node; for previous versions you can use a similar node in Datashapes.
Next step would be to get all tags in the view in the same way, and quarry their host element (there are a few documented ways to do this on the forum - here’s one such method). It may be advisable to use a List.Unique node after this to clean up any duplicate items.
WIth these two lists you can pull the set difference with the List.SetDifference node. You may have to first convert to a compatible object type, in which case an Element.Id node would do the trick on each list.
From there any item which remains needs a tag. Note that if you converted to element id’s you’ll need to convert back to elements prior to tagging.