Counterfeiting of postage stamps remains a problem worldwide.
US Postal Service have a US patent – No. 10,535,211 – that covers an approach based on the application of an adhesive incorporating taggants that can emit luminescence in response to a signal, such as a particular light or electromagnetic energy source. It also covers the use of the taggants within the substrate of the stamp, such as paper, or the ink used to print it.
The abstract of the patent appears below:
Method and system for item authentication and customization
Abstract: A method of illuminating an item is disclosed. The method includes applying adhesive to the item, interspersing a taggant in the adhesive, illuminating the item with an excitation signal, sensing luminescence emitted by the taggant in response to illumination by the excitation signal, and determining the authenticity of the item based on the sensed emitted luminescence. The item can include any item that benefited by authentication and can include a postage stamp. A method of customizing an item is disclosed. This can include preparing a substrate, applying a security feature to the substrate, printing non-customized information on the substrate, receiving image information, and printing the image information on the substrate.
Patent No. 10,535,211
So you can use laser light, and violet light for testing; of course, it is best to go to the post office for verification.