What is a normal ESR reading for capacitors? Well, it all depends on the values of the capacitors (electrolytic).   
Secondary side filter capacitors usually have the value from 220 microfarad to 4700 micro farad. Without second thought, you should expect your ESR meter reading to go as low as possible. If you get any higher than normal reading suspect the e-caps in the secondary side has failed! Checking e-caps in the primary side also important, a slight change in the ESR reading can cause the power to blink, cycling or even produced low power!

Having ESR meter doesn’t mean that you don’t need to inspect other faults in the capacitors. Sometimes a slight bulged, oil leaked capacitors, rusted intermittent broken leads may tested good with ESR meter but failed when power is applied upon the caps. In other word, even though you have the best testing equipment on earth you still need to watch over the physical body of the components you are checking.