Presuming the iPhone is in working condition, and you can unlock it, the easiest way to identify it is to open the Settings app, then select General > About. This page provides a lot of useful information about your iPhone, including its model number and also the model name if you're running iOS 12.2 or later.

For example, you can look at the back of the iPhone for the model number, which is printed in small text. Alternatively, you can check the model number on the original packaging or by using a magnifying glass to inspect the SIM card tray.