This recent image from northwest Illinois shows one of the new dual-polarization radar products now available to meteorologists at NWS Chicago after the recent radar upgrade. The product is called "Correlation Coefficient" (CC) and can be a good indicator of regions where there is a mixture of precipitation types, such as rain and snow. Areas with large CC (near 1, shown in purple above) usually have uniform precipitation (all one type) whereas areas with lower CC (0.88 to 0.98, shown in yellow, orange, and red above) indicate transition regions.
In the image above, valid at 11:17 AM CST on November 9, 2011, the area west of the white line has large CC values near 1, while the two areas circled in white have smaller CC values. This is suggesting a transition from one type of precipitation west of the line, to a mix along the line, and then to another type east of the line. Around the same time of this image, reports of snow started coming in from western Lee, Ogle, and Winnebago counties, while rain continued to be reported east of this line. These reports nicely matched the information we were seeing in this CC radar product.
It should also be noted that the radar beam gets higher off the ground as it gets farther from the radar. So while it may be showing snow or mixed precipitation for a given location, melting may be occurring below the radar beam. Thus the actual transition at the surface may be occurring farther west (in today's example) than shown on the radar.