OpAmp openloop threshold between inputs
the question is what in the datasheet of the opamp I should look to
  find this number?


The open-loop gain of an op-amp at DC can be in the region of 100,000 to 10,000,000. In other words, you need a difference of much less than 0.1 uV to 10 uV to create theoretically zero volts at your output.

However, given that most op-amps might have an input offset voltage of between +/-10 uV to +/-10 mV (an error quantity), it's nonesensical to try and create zero volts at the output by considering the open-loop gain. 

It's also nonesensical to try and do this by considering the input offset error because this is somewhat dependent on temperature and a 1 degree centrigrade change might make a 1 uV to 10 uV change in the offset error and lead to 0 volts on the output rapidly hitting the end stops of the power rails.