There are many criteria for determining drought such as infrared thermal images, normalized deferential vegetative index (NDVI), osmotic adjustment, glycinebetaien

You have to choose some index like Moisture adequacy index or SPI or some other suitable index for categorizing droughts in India.These indices can be calculated,if you have long-term data of rainfall,PET and soil physical characteristics of a network of stations.Based on severity and frequency of droughts, areas can categorized to be prone for droughts of different intensities like mild,moderate and severe etc.Satellite based indices can also be used for categorization of droughts at a larger spatial scale.According to the type of drought i.e.,Meteorological drought,agricultural drought and hydrological drought,the indices and methodologies  to be used also differ.

Drought starts with seasonal deficiency in precipitation over a region for some part of the season but how this deficiency impacts different system depends on many other factors - inherent to the system. So, in every region, a deficiency in precipitation may not always lead to drought.

So, you first have to define what is the objective of your study to delineate the drought proneness. Will u use it for hydrological planning, or agricultural impacts or socio-economic dimension? 

Another question you should be clear is - you want delineate drought proneness of a study area or drought vulnerability. Proneness is only based on frequency (or return period) and intensity of the type of drought you are considering. But vulnerability, which is of more practical value, is broader and need to assimilate different dimensions of drought especially its impact.

If you can define these two aspects clearly, you can progress with identifying criteria of drought proneness. Otherwise, as many respondents have put above, you will keep on getting a endless list of parameters / indices for drought.

You may refer to the publication attached to have better clarity on drought.

Book Manual on Drought Mangement

Certainly rainfall remains the main factor o to the assessment of drought but soil moisture also can provide insight on this phenomenon. Hence among soil scientists textural and structural problems of soil

You are right.Soil moisture or soil moisture related indices are better indicators for assessment of drought and specially, agricultural drought