How do I identify what model

What size is the device? If it is a nexus 7 then you can determine the two versions by looking at the back of the device for a rear camera. if it does not have one then it is the first gen if it has a rear camera then it is the second. You can also determine it by the Version of Android it runs. The first gen only goes up to Android 5.1 and i believe the second one goes up to Android 6. Correct me if i am wrong.