$SU(2)_L$-invariance links charged dilepton and dineutrino couplings. Phenomenological implications are worked out for flavor changing neutral currents involving strange, charm, beauty and top quark transitions in a model-independent way. We put forward novel tests of lepton universality and lepton flavor conservation in $|\Delta c|= |\Delta u|=1$ and $|\Delta b|= |\Delta q|=1$, $q=d,s$, transitions suitable for the experiments Belle II and BES III. Single top production plus dileptons uniquely probes semileptonic four-fermion $|\Delta t|= |\Delta q'|=1$, $q'=u,c$, couplings and further study at the LHC is encouraged. Tests with single top production associated with dileptons and missing energy are suitable for study at future $e^+ e^-$ or muon colliders.