How can I tell if a chip has ESD damage?
The only reliable method I am aware of is decapping the IC (i.e. etching away the plastic housing) and using a microscope. It comes down to looking for visual clues and typical patterns: Overcurrent burns look different than ESD zaps on the integrated structures.

The trouble with ESD damage is that it can be very subtle. You can get anything between a slightly unusual behaviour (e.g. a small change in a MOSFET's gate threshold voltage) and a complete failure of the entire device.