Good question! Subatomic particles, the hadrons and leptons you ask about, are so small they are difficult to detect individually, and elaborate apparatuses are designed for detecting the presence of and measuring the properties of individual particles. Here is a web page
describing a fairly typical, large detector for subatomic particles. These detectors are designed to measure the effects of particles when they pass. They may leave a trail of ionized gas, for instance, or a flash of light when they collide with leaded glass. People have seen with their eyes these flashes of light and sparks one can make with the ion trails. An older technique is to expose a liquid just about ready to boil to high-energy particles, and then to take a picture as the bubbles left in the path of the particle start to expand. Here is a web site
describing one particularly large bubble chamber, along with a photograph of bubble trails left by subatomic particles.
Electrons are the most common lepton, and protons and neutrons are the most common hadrons (see our description of hadrons
. Everything we look at is made up of these things, and so we are looking at them all the time, it's just a question of whether we've seen only one of them at a time. To "see" something with a naked eye you need to shine light on it and observe the photons with the eye (and then one can argue that all you're "seeing" is the photons, a secondary consequence of the electron being there.) Electrons don't weigh very much, so when you shine light on one it will bounce away unless it is held down somehow.
So the answer is no, not to my knowledge, has anyone shined light on just one of these particles and then unambiguously observed the light with their eyes and determined that it came from only one electron or proton. The smallest I remember seeing a picture of is that of a single sodium atom sitting in an atomic trap, fluorescing laser light.
(republished on 07/21/06)