2 improved answer a little
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There are a few things you could do:

  • Compare it with a reflector that is known to be pure white (a purpose made white balance card would be good for this).
  • Take a photograph of it with a light source of (approximately) known colour temperature (e.g. the sun) and spectrum and do a raw conversion with this colour temp and look at the RGB values.

IfThe first method will probably be a lot more reliable as it won't succumb to lighting colour shifts or the vagaries of Raw processing. However, provided it's slightlynot massively off white(in which case you'd be able to see that clearly) it willprobably doesn't matter if it's pure white.

Colour temp is relative so the only really be a problemtime it matters is if you're using it in conjunction withalongside other reflectorslight modifiers which are pure white, so your best betmy suggestion, if this is justthe case, would be to shoot comparison shots with each reflectortry the first option and compare it to see if there's a noticeable shiftyour other modifiers either by eye, or better by shooting images of them under controlled lighting. 

There are a few things you could do:

  • Compare it with a reflector that is known to be pure white (a purpose made white balance card would be good for this).
  • Take a photograph of it with a light source of (approximately) known colour temperature and spectrum and do a raw conversion with this colour temp and look at the RGB values.

If it's slightly off white it will only really be a problem if you're using it in conjunction with other reflectors, so your best bet is just to shoot comparison shots with each reflector to see if there's a noticeable shift.

There are a few things you could do:

  • Compare it with a reflector that is known to be pure white (a purpose made white balance card would be good for this).
  • Take a photograph of it with a light source of (approximately) known colour temperature (e.g. the sun) and spectrum and do a raw conversion with this colour temp and look at the RGB values.

The first method will probably be a lot more reliable as it won't succumb to lighting colour shifts or the vagaries of Raw processing. However, provided it's not massively off (in which case you'd be able to see that clearly) it probably doesn't matter if it's pure white.

Colour temp is relative so the only time it matters is if you're using it alongside other light modifiers which are pure white, so my suggestion, if this is the case, would be to try the first option and compare it to your other modifiers either by eye, or better by shooting images of them under controlled lighting. 

1
source | link

There are a few things you could do:

  • Compare it with a reflector that is known to be pure white (a purpose made white balance card would be good for this).
  • Take a photograph of it with a light source of (approximately) known colour temperature and spectrum and do a raw conversion with this colour temp and look at the RGB values.

If it's slightly off white it will only really be a problem if you're using it in conjunction with other reflectors, so your best bet is just to shoot comparison shots with each reflector to see if there's a noticeable shift.