Determining Operational Amplifier Offset Voltage
When dealing with real world op-amps there are two major offset parameters: input offset error and output offset error. Generally, these are combined into one parameter because they are difficult or impossible to separate in practice. So, you will rarely read about output offset error, even though it is a real metric of the design.  Instead, output offset and input offset errors are combined into one characteristic, and it is called "input offset error". 

How to measure input offset error? (i.e. The  total of input &amp; output offset errors as explained above.) The easiest, most practical way is to configure the op-amp as a unity gain buffer ( connect output to the minus input). Apply a voltage, within the common mode range of the op-amp, to the positive (i.e. non-inverting) input. The difference between the applied test voltage and the output voltage is virtually totally attributable to the input offset error of the amplifier.

You can connect a DVM set to its lowest range between the output and the NI input and read the actual input offset voltage value directly, assuming you have a sensitive enough DVM. Vary the test voltage across the common mode input range to find the worst case. 

I'm not a SPICE guy, but I'd expect you can do the same thing in a SPICE model.