What is the best way to find the specifications of a plc that you are going to work on...?

By Calculating IO'S  (PLC IO CONFIGURATION) and by going through the Application Requirement such as Communication Protocols/Modules,  Power supply Requirment,Processor Capability,Memory Requirement and Ruggedness of PLC depending on Site Conditions.

All PLC's has got its own data sheet showing its specifications. Like I/O counts, communication Protocol, types of applications etc. its available with the plc or in the concerned company website.