How an op-amp determines its output without a reference point?
Normally you can assume midway between the supplies. 

It actually doesn't matter much for most high-gain op-amps since you can translate the input offset voltage into a change in output offset voltage by multiplying by the gain, and that number is usually many times the power supply voltage(s). 

For example, take an LM324 which has a gain of 100,000 typically and an offset voltage of +/-2mV typically. That translates to +/-200V at the output, so the particular reference point you pick on a +/-9V supply only makes a difference of less than +/-90uV at the input. 

Real op-amps also have finite CMRR and usually we assume the inputs are also at or near the midpoint of the supplies. Away from that you'll effectively have a change in input offset voltage (sometimes quite significant). To be sure, check out the datasheet conditions for measurement of CMRR and Vos.