I have a power supply like this:
http://www.mcccomputers.com/shop/573...-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.
In the picture you linked to
In the picture you linked to the polarity is shown just below the model number . The center of the plug is positive and the outer sleeve is negative . You should test it with a multimeter if you have one . This will tell you the polarity as well as the actual voltage . It’s a good ideal to check the voltage coming out of any power supply before you use it .
Power Supply question
You can also look at the wire…“wall wart” wiring:
Black = Negative
Black with white stripe = Positive
If there’s no tip on it
If there’s no tip on it anymore the only way to be SURE which is which is by hooking a meter up to it, if it comes up as negative voltage then you’ve got it backwards. If you don’t have a meter I suppose you could hook it up to an LED with a resistor (any diode would work, but an LED has a built in “it’s working!” light 
)