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By referring to your circuit, you tie up Input + and Input - together. Then give a voltage source, let say VDC to ground. The output + and - also you tie up together and leave floating. Now in the ADE in Cadence, choose DC analysis and under operating options, you choose Design variable and it will ask you to point a variable. You click at the VDC. Then you sweep the VDC from low to high voltage power supply. Then you plot the input vs the output. From the graph you will know what is the common mode range of your diffamp.
As pillar_chen write, first you shuold build the opam as a voltage follower ,second chang the input dc voltage of the follower and do ac simulation under diffirent dc voltage, last you will find out that when the input dc voltage lower or higher than two certain voltages, the follower can not work correctly. The difference of the two voltage is the input/output common mode range.
Sorry for the confusion above. What i have given is the method for CMRR measurement. Actually, to measure the common mode range, use the unity gain buffer configuration (voltage follower as chen said). Sweep input positive dc and output connect to input negative terminal. Then probe the input and output.
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