How to tell flat flexible cable(FFC) alignment when inserting?
The header designed to accept it is an 8 pin FH52-8S-0.5SH, with contacts on top and bottom.


Check the datasheet available here. This connector has contacts on the bottom only.


  In general, is there a rule of thumb that contacts should always go up
  or down or does it just depend on the design?


It depends on a design. Most connector series include both variants with contacts on top and on bottom, providing designers with flexibility in mechanical orientation of the components. Many series also include variant with contacts on top and bottom. That does not mean you can insert FPC either way. Although it is technically possible, most likely it simply means 2-sided FPC is used to reduce the overall width.