When searching for a PCB, the most common information that will be in the database are Part Numbers.  The part numbers will either be printed on the PCB, or on a specific component.    

 

If you're not that lucky, then the next thing will be to identify specific physical characteristics of the PCB, such as number of boards, how they are grouped together, the dimensions of a board, etc... 

 

If the PCB has no markings, hopefully the contributor included components that are unique to the PCB or  documented the location of specific parts that would make the PCB unique.

 

The information on the PCBs may not be complete, so you can also browse the list of PCB's by not providing any parameters.