Exif uses "GPS" on the names, so I would expect WGS-84.
In any case, the Exif 2012, page 73:
GPSMapDatum Indicates the geodetic survey data used by the GPS receiver. If the survey data is restricted to Japan, the value of this tag is 'TOKYO' or 'WGS-84'. If a GPS Info tag is recorded, it is strongly recommended that this tag be recorded.
so you should check that field, and I would expect to see always WGS-84.