This app was mentioned in 3 comments, with an average of 2.67 upvotes
Leafspy is an app that can poll a bluetooth OBD2 dongle to find leaf specific information, such as battery SOC, capacity, cell voltage, temperature, etc.
https://play.google.com/store/apps/details?id=com.Turbo3.Leaf_Spy_Lite
https://play.google.com/store/apps/details?id=com.Turbo3.Leaf_Spy_Lite :) I'm wondering because I've ran mine for 65K km, and my soc is not even below 90% - so far as I understand, that means it's pretty close to "factory defaults". With a 2013 Leaf.
I just bought an end-of-lease 2013 S model in the Seattle area for $10,500 + tax. If it's not a Nissan dealer, they are going to know very little about the car. Make sure they give you a CarFax report!!
Then buy one of these from Amazon:
The free version of Leaf Spy will work fine for battery health inspection:
https://play.google.com/store/apps/details?id=com.Turbo3.Leaf_Spy_Lite
Look for the "SOH" in Leaf Spy. It is a percentage that tells you the remaining capacity of the battery pack. My 2013 Leaf with 23k miles is at 93%. That seems pretty typical from what I've seen.
Also, Leaf Spy will report number of Quick Charges (QC) and L1/L2 charges. Your 2012 might not have the QC port, but if it does, there's a strong correlation between number of QCs and lost battery capacity.