| Using a voltage tester, you can test each black wire and go acros the bare, only one will show voltage. There are also touchless testers that indicate which wire is hot.
If you are changing fixtures, you can remove one of the sockets from an old fixture, strip the leads about an half inch, install a small light bulb in the socket , and use it as a tester.
Again, touch one of the socket wires to the bare, and the other to the wires in the box, the hot one will cause the light bulb to light up.
Before doing any testing, be sure to know which circuit breaker or fuse shuts off the power, so that while working with wires you can work safely. When ready to test, keep the wires clear of each other, and the bare wire, and the metal box. Cap the wires with wirenuts , and only remove just before testing.
Once you find the hot wire, use some white tape to identify the other wire as the neutral wire. | 
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