Have gluons actually been detected, and if they have, how?
Yes. Gluons were first conclusively proven to exist in 1979, though the theory of strong interactions (known as QCD) had predicted their existence earlier.
Gluons were detected by the jets of hadronic particles they produce in a particle detector soon after they are first created. In particular, at the PETRA collider at the DESY facility in Hamburg, Germany positrons and electrons were collided producing an intermediate photon that then produced a quark and an anti-quark as it decayed. This process results in two jets of hadronic particles as the quarks form hadrons. Sometimes one of the final-state quarks radiates a gluon just before it "hadronizes" (that is, forms into hadrons such as protons, pions, neutrons, etc.). This gluon will form a jet of its own, producing an event with three jets, instead of the usual two.
Four different detectors at PETRA in 1979 reported observing the three-jet events with just the right frequency and just the right characteristics to be consistent with gluon production. Later experiments at CERN and Fermilab confirmed these results.
One of the original papers that reported the three jet events is in Physical Review Letters, volume 43, page 830. A more detailed discussion can be found in the Cahn and Goldhaber book "Experimental Foundations of Particle Physics."
Brent Nelson, Ph.D., Research Fellow, University of Michigan
'The strength and weakness of physicists is that we believe in what we can measure. And if we can't measure it, then we say it probably doesn't exist. And that closes us off to an enormous amount of phenomena that we may not be able to measure because they only happened once. For example, the Big Bang. ... That's one reason why they scoffed at higher dimensions for so many years. Now we realize that there's no alternative... '