There are many ways to see invisible ink, but the most important thing is finding a light source that emits a specific wavelength. Most people use UV or blacklight bulbs, but some also have regular lamps that work. You can buy specialized glasses to filter out certain wavelengths and make invisible ink visible.
During World War II secret messages were sent using invisible ink. The ideal ink would not smell, be detectable by iodine and ultraviolet light, be heat resistant, and only reveal itself when sprayed using an atomizer. The Allies and Axis laboratories competed to create this holy grail invisible ink.
Some types of invisible ink can be developed with a simple acidic liquid, such as lemon juice. Other acids, such as vinegar, milk, apple juice, onion juice and even some bodily fluids like saliva and urine can also be used to write hidden messages. Some of these inks are created by heating them, which makes the inks appear darker on paper.
Other invisible inks use fluorescence, and require a light that emits the right color of light. These inks are usually used to write secret messages on cards for poker games or other games. In order to make these inks visible, they need a light source emitting blue. Most people who use invisible pen ink wear sunglasses that block the wavelength of light required to make the ink visible.