To test the IC I would try these steps:
* Use a power supply of the minimum recommended voltage.
* Create a resistive substitute for the transformer.
* Apply a feedback voltage to pin C.
* Test whether pin D provides an On-and-Off path to ground.
To test the remainder of the circuit, you can make an oscillator to turn the transformer On and Off. Attach a resistive load to a secondary. Observe output waveforms to see transformer performance. (Your schematic appears to have no load attached to the output. It may require a load in order to operate properly.)
The transformer is able to produce a high-voltage spike when current is abruptly stopped. The spike can be hundreds of volts. Normally the snubber network absorbs the spike. But since your schematic has the snubber network crossed out, then there's a chance your IC was exposed to a high-voltage spike within the first one or two cycles of use. It may be ruined.
Sometimes I do an ohmmeter check on an IC. From one pin to another. Although there are dozens of possible combinations, I don't perform all of them. If the IC is good then it shows different ohm readings between its various pins. But if ohm readings are all the same, then it tells me the IC probably has fused innards, meaning it is destroyed.
I prefer to do such ohm tests at the highest ohm range (x 1k). I prefer to use my analog VOM which runs from a 1.5V battery. My digital meters contain a 9V battery and IC's don't always tolerate such a voltage difference applied to its pins.