How to identify an unmarked PIC?
The pin layout is largely compatible in Microchip's line and there is a device ID (part number and silicon revision) factory programmed at a fixed location in newer PIC devices.
Programmers such as PICKit attempt to read the device ID. As far as I know, Microchip does not publish an official exhaustive list of device IDs, however those who create programmers use that information so the information has been gathered- for example this list.
See also the ICSP Programming Guide for the expected locations.