How do I tell which wires are Red/Positive and Black/Negative on these fans?
A quick search does not bring up this particular series on the Delta website. That said, this is a 4-wire DC brushless fan and there are some good resources available on understanding how these are wired up and controlled.

The typical wiring pattern for these fans is GND/12V/SENSE/CONTROL. Finding the ground wire should be easy enough with your multimeter. 

Take a look at these websites if you want to learn more. Perhaps someone with direct experience on these fans can chime in.

http://pcbheaven.com/wikipages/How_PC_Fans_Work/
http://www.hardwaresecrets.com/anatomy-of-computer-fans/2/