Lighting, Light Fixtures, Ceiling and Exhaust Fans - How can I tell which wire is which?

I have a dimmer switch with just two black wires on it -- how can I tell which is which?

Also -- I want to put this dimmer on a wall switch where there are 2 switches. It seems like someone once told me that I needed to use a different type of dimmer when there were two switches, as opposed to a single wall switch. Is this true? Or can I just put a regular single-pole switch on one side (I only want one of the lights to be on the dimmer)?
[I probably explained that badly -- sorry -- I hope it's at least understandable! :o ]

You would be wise to go to the local homedepot/lowes/ace hardware and pick up a pocket electrical tester(not expensive). When the power is on and the light switch is off, the tester will will make a noise, and flash a light when you touch the plastic coating of any live wire. Make sure the light switch is turned off.

You will need to buy a dimmer that is designed for a 3-way operation. These will have an extra connection point to allow the multi way operation. There are two type of 3ways for dimmers. The cheaper ($5-10) way allows one switch to turn on/off/dim, the other switch will only turn on/off no dimming. The more Expensive way ($15+) will allow both locations to dim the light.

Without major rewiring all the lights on the switch will dim.

`Brett

Thank you Brett! That answers my question to a T and is exactly what I wanted to know (apparently I didn't explain it too badly :) ). Fortunately I can go the cheap route -- the other switch goes to a closet - no point dimming the lights in there!

Re-reading your post, I realize what you are asking better now.

You just need a standard dimmer. I thought you were stating you had 2 switches controlling the same light. You have 2 switches in the same box. Just get any dimmer you like. You will probably be limited to 500 watts of light bulbs when they are ganged in the same box like this.

You dont need a 3-way dimmer.

Sorry, I misunderstood your post with the new info on your last post.

As far as the two black wires, it doesn't matter. Your old switch has two wires + a ground(maybe). Just disconnect those two wires and connect them to the dimmer wires. It doesn't matter which one goes to which wire(except the ground). The ground always goes to the green or bare.

OH!!! Well then.... that's a horse of a different color! I like that answer even better! Thanks again Brett!

... dimmer is hooked up -- working fine! Thank you very much for your help, Brett! :D

It was my pleasure.