One such observable is the azimuthal decorrelation of the two leading jets, produced e.g. at hadron colliders such as the Tevatron, which is closely related to the pattern of additional QCD radiation. In fact, without this further emissions the two leading jets would be oriented strictly back-to-back; the more radiation occurs, the more this correlation is attenuated.
Another way of checking the correct amount of QCD radiation is to test whether it produces the right amount of hard radiation, which ultimately manifests itself as jets, sprays of particles hitting well-separated regions of the detector. Typically this is a regime of QCD where Monte Carlo event generators do not perform too well; only recently methods have been developed that overcome this deficiency. To test them, one of the standard processes is the production of a Z-boson, in addition with a varying multiplicity of jets.