The most obvious way to tell them apart is of course wingtips - J-16D/J-15D are the only two chinese flankers using wingtip pods; otherwise, if you see pods on chinese flanker from any range - it's Russian flankers in chinese service.
But otherwise (adding to @Tomboy) there are many differences - visibly different radar(smaller cone), conpletely different configuration of antennas around the airframe(below cockpit and behind it; stabilizers).