Old cloth covered wiring

I bought an older house and am trying to update some of the switches and outlets. What is throwing me off is the wiring inside is older, cloth covered. Some of the wiring coming into the switch/outlet boxes are white/black, some are black/black. How can i find out which wire is which if I'm not sure? And how can I check to make sure I am doing it right, aside from plugging something in and seeing if it works? Thanks.

 

Black and white should still apply to cloth wiring. White is neutral.
A rule of thumb. When the switch is off, there should be no power to ground at the light. If there is, then you switched the nuetral, which is a no no. Although the only problem is servicing the light. The light will work fine regardless. Switching the hot keeps you from getting zapped while working on the fixture with the switch off.

 

Most of the wiring in my house was knob and tube (the cloth covering you describe is called loom) and it was all black, no white at all either, so I can understand your dilemma. The best thing to do would be to replace it with modern Romex wire. If rewiring the whole house is not feasible and you just need to find hot and neutral then you need to have a grounded outlet somewhere and a voltmeter. Find out if you have an actual grounded outlet, and not just an outlet masquerading as one. Plug one end of the voltmeter in the small hole on the outlet and one in the ground hole, if you get a voltage reading near 120 then its grounded. I used the ground from a three prong extension cord to check some wires that weren't very accesible to determine which was the hot one. If you don't have a grounded outlet anywhere I'm not sure how you can determine which one is hot.

 

In general of course, you should connect the new receptacles and switches as the old ones are connected. This rule is especially important if you're not sure what is what. Trying to "fix" something that you don't fully understand is the source of many calls to the electrician. Most of the time, it didn't need fixing.

Having said that, I note that there are a lot of ways to screw it up even if you think you are wiring it exactly as it was before.

If you have two black wires, one hot and one neutral, a $15 "tick" tester is an excellent tool for telling them apart. This is readily available at Home Depot and similar stores. No ground is necessary to use it.

 

Just a little tip. Take a flashlight and look way back into the box where the wires enter. Many times the old insulation white has come off but further back in the box it is still visible. Another way to test as has been said here is a tick tester. Also another good trick is with a voltage tester use a nearby ground. Any heating pipe is a good way to find a ground nearby to test from.
Good luck