  We consider a tunnel junction between two arbitrary non-linear systems in any
dimension, which can be different. We show that the tunneling charge can be
detected using three alternative methods based on current measurements. Besides
being technically easier compared to noise measurements, these methods present
valuable advantages: they do not require the knowledge of the underlying
models, and some are accessible in the experimentally convenient low-voltage
regime, where heating effects are reduced. The first method is based on the AC
conductance, while the two others are based on photo-assisted current (PAC) and
can be implemented for any time-dependence of the tunneling amplitude. These
are promising for edge states in the regime of the fractional quantum Hall
effect (FQHE): the Hamiltonian does not have to be specified and can
incorporate non-universal interactions between the edges, and it is more
convenient to use an AC gate voltage rather than an AC bias. These methods
apply for instance to weak barriers in 1-D systems,
Superconductor-Insulator-Normal (SIN) or graphene-like structures.
