how do i know which is positive?

Well, I didn't find an answer to this anywhere, so I decided to ask.
My plan is to do what SilverSportTrac did in his post about LED's and get some bulk LED's, some resistors, and a breadboard from radioshack to replace my dome light.
Using a calculator online I know what type of resistors I need, and that the longer wire on the LED's will be the positive one.

So, my question is, if I do this, how do I know which wire going to the dome light is positive or negative? This setup, since it's not a little 4LED bulb, can't easily be turned around, so if I get the wrong one the LED's wont work.

Any help would be appreciated.

 

[wired_af]

Use a volt meter if you have one.

 

[jared ajlouny]

where do u get leds and how much, i want to get one for my third brake light

 

[Evicerate]

Weird Af, thank you. I feel dumb now, but thats how I learn.

Jared, they have them on ebay for my 94, if your highmount's the same I guess they'd have them. Price varies, maybe $3-9?