You shoot RAW, perhaps (at least I hope you do). That means you need not worry about setting white balance while shooting.
So how do you set white balance in post-production?
Ideally, you include a grey card and use the dropper tool in Lightroom (if that is what you are using) to take a neutral reading off this. But if you do not have a grey card in the photo?
Look at a student who kindly agreed to be the subject of a test picture. One: the original photo
Two: after I take a white balance off the eyeballs:
Three: as a personal preference, since I like warmer light I then always drag the colour temperature slider to a slightly higher temperature (a slightly warmer light):
And hey presto – done.
This is quicker than doing it on the camera, and more accurate, and you do not waste your subject’s time.