In this posting I explained last month how a hail spike on radar was a near 100% indicator of large hail. We have one (arrow) on the radar in eastern Iowa right now.
This image was captured at 6:29pm CDT and the largest hail was falling between East Amana and Homestead, Iowa (circled).
UPDATE: 2" diameter hail confirmed 2 mi. south of Amana, exactly where the radar indicated.