UV-C light has wavelengths between 200nm and 280nm. Its germicidal properties are capable of inactivating a wide range of microorganisms, including viruses and protozoans, by scrambling their DNA.
There are UVC LEDs that are capable of producing germicidal light to sterilize surfaces. But they are expensive to produce. If a product is super cheap, there is a high probability that the UV-C lamp is not genuine. Just high cost is not a determiner for the genuineness of UVC sterilizer. UV-C is am electromagnetic radiation that is invisible to naked eyes. You can use a spectrophotometer to measure the wavelengths. They are expensive, so many you can consider borrowing or renting them.
You can improve your chances of finding a working UV-C sterilizer by buying FDA approved UV devices. With other certificates and paperwork that comes along with the sterilizer.The most common and easiest method is using UV-C light measuring meters – radiometer calibrated to detect UVC, which can help you know if the UV-C light is in its germicidal frequencies.
You can also use samples of certain minerals that exhibit the property of fluorescence when radiated by UV-C light. It is essential to put on personal protective kits, Use proper safety google that block UV light and be careful to ensure you are not exposed to UV light. It is detrimental to human skin and eyes and can lead to skin cancer.
You can test it by its effect on the bacteria you want to kill. You can get a couple of Petri dishes, expose one Petri dish to the UV light, keep the other as is, and then check for the difference in the colonies in both Petri dishes to see how efficient the UV light is inactivating the bacteria. Viruses require much high does and are harder to validate. You should take care of the UV dose by ensuring the time and distance to exposure to the type of bacteria you specifically want to kill.
You can also use som disposable indicator strips available in the market with a substrate with a photoactive ink in which response to the kind of UVC does receive and change color. You can also explore the spore strip and incubate two pieces, one exposed and average for 48 hours and check for the specific color change.