As fluorescent brightener is known to more and more people, UV flashlights become popular for the UV led torch light can detect florescent agents. So, how to detect fluorescent agents in disposable paper cups? It is quite easy.
The below tests are carried out with a common 365nm UV flashlight (not Tank007).

How to test page and disposable paper cup

Disposable Paper Cup
Put the paper cup into camera obscura, turn on the UV flashlight, shine the light to the cup, if no color changes in the cup, it indicates that the cup does not contain fluorescent agents. If fluorescent reaction occurs, there is fluorescent agents in the cup.
A4 white paper
Like testing the paper cup, put the white paper to the camera obscura, turn on the uv light, if the paper turn to bright blue, it means the paper contains lots of fluorescents agents.
So, are the tests above really right? May be.
In fact, not all UV flashlights throw pure UV light. How to tell UV light and other lights? It is easy. UV light is invisible, so you cannot see it. But some lights from UV flashlight include blue light, which can be seen. That’s why some people find that, even the paper that does not contain fluorescent agent shine blue light under UV light. So, it is not reliable to test fluorescent agent with common UV flashlight, you need professional UV flashlight.
What kind of UV flashlight is best for detecting fluorescent agent
According to many tests’ result, UV light with wavelength between 254nm to 365nm is best for detecting fluorescent agent. If the paper cup, A4 paper, facial mask contains fluorescent brightener, fluorescent agent reaction will happen under UV light.

Can fluorescent agent cause cancer

This saying need to be tested by authorized organization. Now, no test result shows fluorescent agents can cause cancer. But it is harmful to human body, therefore, the use of fluorescent brighteners in food, food wrappers, napkins are prohibited .