I have a power supply like this:
http://www.mcccomputers.com/shop/57...ly-adapter-rwp480505-1-02477800-5v-1a-13w.jpg

Is it okay to assume that the left wire is negative and the right wire is positive?
I don't have a connection in the end as shown in the picture. I need them open to connect to a terminal block. Thus, I need to know which is positive and which is negative.

 

How could you assume that? The wires can be in any order, the only thing that is known is the connection at the plug. If you don´t have it, why not just measure the voltage? Or connect a LED with appropriate resistor and try which is which if you don´t have a meter.

 

As Kubeek says, the diagram on the label indicates that the center terminal of the plug is positive, and the barrel is negative.

You can make a simple polarity checker out of a red LED, a green LED and a 470 Ohm resistor, like this:

You could also use a bi-color LED if it only has two leads; the 3-lead bi-color LEDs won't work. With the bi-color LED, if the input is AC, the LED will appear orange.