Ali , your question is very practical , and this is where , most of the times , we miss our bus by a mile  , by not able to take stock  of the tests to be carried out  to meet the requirements necessary for  getting the response of biofertilizers .    The soil  inoculation response of  microbes  will only be effective , unless , there  is ample energy stored inside the soil for proliferation of inoculated  microbes through organic carbon stock . We need to sure a  minimum level of organic carbon present in soil , depending upon the soil types and crops grown ( A researchable issue) .In addition , initial  fungal an d bacterial count of the soil would give an indication  , how live a soil is microbiologically?. the changes in population following the inoculation will further give us the  necessary clues for their response. We can also go for certain soil enzyme analysis in terms of , urease , dehydrogenase activity , phosphatases( Both acid and alkaline phosphatases) etc will further supplement the  reasons for response of biofertilization .

you must run parallel sets by using fumigated sterile soil and normal soils in some pot studies to see if the organism is effective. Sometimes the native populations mask the effect as there is a lot of competition with native flora. Testing native flora is very difficult . Heavy dose of  biofertiliser may give additional advantage.