We have all been there. Beautiful subject in beautiful light but no white balance reference to give us a tarting point in post. Oh, and did I mention that there isn't a 18% grey card around either?
Now you may be happy with the color the way it is but what if you like to eliminate the color cast from the lighting?
Here is a trick.
If you have a watch with a metal arm band, you're in luck. Just take a picture of your watch.
Later in post, use the watch as a reference point for the white balance. Make sure you click on an area of the watch which is illuminated by the ambient light and not a reflection of something with a color. You don't even have to take your watch off.
OK, this is no 18% grey card, but it will help you in a pinch. From this point on you can manually adjust the white balance to a point that you are happy with.