That i-MiEV looks very nice, and there is a $4000 'grant' as well?
Battery degradation is a complete non-issue at this point in the car's life, so I wouldn't worry about it. Any serious battery problem will be covered by warranty until 2022 (or maybe even 2024?). FWIW, this car has the 'better' battery that was introduced in late-2012.
If you would like to check the battery capacity, you would need a Bluetooth OBDII device, specifically a genuine (not knockoff) OBDLINK LX or MX http://www.obdlink.com
and a free Android App called CaniOn or another app called EVBatMon for Android or iOS (I don't know how it performs under iOS) - both apps have a readout of what the car thinks is the battery capacity in ampere-hours, and I would expect a reasonable reading to be around 37Ah-41Ah, but it's not all that important, as many of us don't even notice any degradation in our daily driving.Edit:
Let me emphasize that the car should come with a Remote which can control both the timing of the car's charging as well as pre-heating or pre-cooling the car while it is plugged in. If this Remote does not come with the car that would be good grounds for negotiation, as a replacement Remote can run close to $1000 with dealer programming.