How can I check whether white fabric will cause a color cast as a reflector?

There isn’t a simple, absolute way to prove a fabric is “pure white” in all lighting. Many white fabrics contain optical brighteners/fluorescent dyes, so their apparent color can change depending on the light source, especially its UV content.

The practical test is to evaluate the fabric under the same light you’ll actually use. Compare it to a known neutral reference such as a white balance card, or photograph it in the intended setup and check whether it reflects neutral light. If you shoot digital, use that setup to set white balance; if you shoot film, make a reference frame and correct from that when printing/scanning.

In practice, it usually doesn’t need to be perfectly white—just close enough that it doesn’t introduce an obvious shift. This matters most if you’re mixing it with other modifiers that are known to be neutral. If it looks noticeably different from a known white card, choose another fabric.