There are three root-level Geo objects in the payload – two of which are supported and may be used to identify geotagged Tweets (see the Geo objects documentation for further reference):
coordinates - relies on exact location [long, lat]
place - a specific, named location with corresponding geo coordinates
geo, is deprecated as documented near the bottom of this page. You should use the coordinates field instead.
Unfortunately, we can’t share exact figures around the percentage of Tweets that contain geo (place / coordinate).