I harvested a PCB from a tecan cavro multichannel pump (pdf manual attached). I want to control just the solenoid 3 way valves via arduino. One PCB seems to control all 8 solenoids which are 24VDC (current I'm not so sure). From what I can see two darlington transistors (shift registers?) which control 4 valves each. My question is, how would I go about working out which of the pins to provide the 24VDC to and which pins I can switch the solenoids with?

I've attached an image of the PCB .
Any help would be greatly appreciated.

 

The information on connecting the 24V supply to the connector is detailed in section 2 in the manual you posted. The module is controlled by serial inputs via the communication inputs via RS232/485 or CAN, again detailed in section 2. Section 3 gives a comprehensive description of the protocol and commands. If you want to re-purpose this module, you are going to have to do a lot of reading. There is no short-cut.

 

My question is, how would I go about working out which of the pins to provide the 24VDC to and which pins I can switch the solenoids with?

It appears that each solenoid is activated directly by an output of the shift register which is an open drain mosfet. Each 2 pin connector should have a constant 24 volts on one of the pins. The other is the pin that is pulled to ground to activate the solenoid.
Those DN number surface mount components I believe are dual diodes for EMF suppression across each solenoid.