Dwarf stars range from yellow dwarf stars, such as the Sun, to red and white dwarf stars. Of all the stars in the sky which are visible to the naked eye, every single star is bigger than the Sun. Thus, dwarf stars are clearly hard to see, but there are other ways of detecting dwarf stars than by their light.
A red, yellow or white dwarf star can theoretically be seen by the light it emits. A brown dwarf can only be seen by the light it reflects, because it does not emit any light. In fact, strictly speaking, brown dwarf stars are not stars at all, but fall halfway between stars and gas giants. They have the fully convective surfaces and interiors of a star, but they cannot sustain hydrogen fusion in their cores.
To be visible to the naked eye, a dwarf star must have a magnitude of less than 6.0, although the dimmest of these stars need perfect viewing conditions to be seen. Many white dwarf stars, such as Epsilon Eridani, are bright enough and close enough to be seen with the naked eye.
Red dwarfs cannot be seen at all with the naked eye. The closest red dwarfs, such as Barnard’s Star, can be viewed through binoculars or a telescope. With strong enough telescopes, they can sometimes be detected directly from their emitted light.
In theory, there should be no limit to the light-gathering properties of a large enough telescope. However, very large telescopes are extremely difficult to make, and even a single imperfection can make them useless. Telescopes can detect more dwarf stars than the naked eye, but only a small fraction of dwarf stars can be seen this way.
Some dwarf stars, such as Barnard’s Star again, radiate more strongly in the infrared, a different electromagnetic frequency than visible light. Telescopes have been built to observe in these spectrums as well, but their resolution and their size are also limited.
Sometimes a dwarf star in a binary system can be visually separated out from its companion star through a telescope. If the main star is too bright, a coronagraph can be used to block out its core light and expose fainter companion dwarf stars.
The existence of dwarf stars can also be inferred by the variation of light during eclipses, when one star in a binary pair is hidden behind the other. The light variability can be several orders of magnitude.
A dwarf star can be separated from its larger companion star through spectrographic analysis. Stars in orbit around each other are sometimes moving less or more quickly away from the Earth than at other times. That means that the Doppler shift in each star’s spectrogram will be different.
Finally, dwarf stars have a high enough mass to exert the kind of gravitational pull which can cause visible effects, usually an otherwise inexplicable wobble. Thus, the existence of a dwarf star can be inferred from the gravitational pull on its companion star.
This is the primary method used to detect brown dwarf stars. It is also the main way to detect exoplanets.