It also depends on what light you're using, like flash or other. I like to keep a gray card with me (WhiBal from www.Rawworkflow.com) and include it in a test photograph, under the same light that is falling on the background. After white balancing* the gray card in your editing software, the cast will change on the background and let it be white, assuming as the others said, it is lighted bright enough to be a real white.

Three alternative methods:
* one, click the white balance eyedropper onto the gray card in the picture and then copy the white balance settings only, from your test shot to your good shot(s) that were taken in the exact same light.
two, put the gray card in a part of the composition (under the background lighting) where it can be cropped out, white balance to the gray card same as number one. Then crop the gray card out of your test shot.
three, with the gray card in the same light as the background, manually meter the gray card only, with whatever settings you desire, then remove the gray card and take the picture(s).