Possibly the only way to ensure you have something absolutely neutral in the scene is to get a white balance reference card of some kind. 18% gray cards can work, but knowing they're color-neutral is another thing. There are expensive tools like this, such as the WhiBal or the ColorChecker Passport. And if you have the camera set in RAW, you simply take the picture twice, one with the reference card in the shot and one without, and after you've white-balanced the shot with the reference, you then duplicate that setting on the shot without it (i.e., use the setting synchronization feature in Lr or ACR).

However. This only makes your white balance match the color the object is in white light. Not necessarily the color it looked like in the lighting conditions you were shooting in. Sunsets, for example, warm any scene with a gold glow, and white balancing with a reference is liable to lose you that golden quality that marks it as a sunset scene.

So, getting an "ideally neutral" value to white balance from may not actually be the ideal way of dealing with white balancing. Sometimes adjusting "by eye" in the software until it looks right will work better. References are when you need the color of the object to match that of the image exactly--and most typically under studio conditions where once you've got everything set up, all the subsequent shots can use the same correction.

Possibly the only way to ensure you have something absolutely neutral in the scene is to get a white balance reference card of some kind. 18% gray cards can work, but knowing they're color-neutral is another thing. There are expensive tools like this, such as the WhiBal or the ColorChecker Passport. And if you have the camera set in RAW, you simply take the picture twice, one with the reference card in the shot and one without, and after you've white-balanced the shot with the reference, you then duplicate that setting on the shot without it (i.e., use the setting synchronization feature in Lr or ACR).

However. This only makes your white balance match the color the object is in white light. Not necessarily the color it looked like in the lighting conditions you were shooting in. Sunsets, for example, warm any scene with a gold glow, and white balancing with a reference is liable to lose you that golden quality that marks it as a sunset scene.

So, getting an "ideally neutral" value to white balance from may not actually be the ideal way of dealing with white balancing. Sometimes adjusting "by eye" in the software until it looks right will work better. References are when you need the color of the object to match that of the image exactly--and most typically under studio conditions where once you've got everything set up, and all the subsequent shots can use the same correction.