A modern equivalent is SmartWater, a clear fluid that a) fluoresces under UV b) is extremely difficult to wash off and c) contains a unique code for forensic identification.<p><a href="https://en.wikipedia.org/wiki/SmartWater" rel="nofollow">https://en.wikipedia.org/wiki/SmartWater</a>
Glo Germ powder is a consumer equivalent of this:<p><a href="https://www.amazon.com/gp/product/B00FE1RQA2" rel="nofollow">https://www.amazon.com/gp/product/B00FE1RQA2</a>
How exactly would it be used? Presumably you're using it to mark someone you don't actually know, but then you need to actually go looking for the trail, but how do you do that if you don't know who the spy is?