Measure sensitivity of ADC
Is there a standard method for doing this?


Yes (each manufacturer has slightly different methods), if you want to measure noise, then ground the inputs. INL and DNL will require a much more involved setups (shown below). Most methods involve a signal generator with a ramp signal. The ramp signal is compared to the output of the ADC and the INL and DNL are calculated.




Source: https://www.analog.com/media/en/training-seminars/design-handbooks/Data-Conversion-Handbook/Chapter5.pdf


  How would you go about measuring accuracy of any ADC?


Depends on the parameter that needs to be measured and the application. In my case, I mainly care about INL. I use a 24bit ADC for a control application, INL can create error in the feedback loop. If you need absolute precision, in my opinion INL is one of the better things to look at. If relative precision is important DNL might be a better thing to look at. Some applications require low noise. 

But you don't need to measure the accuracy of the ADC, you only need to understand how the manufacturer measured/tested the performance as it is guaranteed in the datasheets of most manufacturers. If you design with an ADC and don't get the performance listed in the datasheet, that would be a good point to talk to the manufacturer. 

If I did have to measure the performance, I would go to the manufacturer and get the application note for testing ADC's and use their methods and circuits. 


  Would high ratios of voltage divider help to generate a low noise low
  voltage signal in µV range to feed into the ADC?


If you want to design a uV to nV system, the noise of all components needs to be considered, this means doing a noise analysis. A good resource for this is Noise: three categories: Device, Conducted and Emitted. Noise bandwidth and filtering are essential concepts to understand how to attain low noise.


  Wouldn't Johnson noise stop one from using high value resister in this case?


A 24-bit ADC with a 5V span has 298nV per bit, A 500k resistor with a 5Hz low pass filter has 201nV-rms of noise. That same resistor with a 500Hz filter has 2uV-rms of noise. So filtering becomes important.