  We propose possible probes which could be used to demonstrate experimentally
the existence of the bulk and the formation of a traversable wormhole purely in
terms of boundary operations only. In the two-dimensional Einstein-dilaton
gravity, the traversable wormhole is realized by turning on a double trace
interaction which couples the two boundaries of the AdS$_2$ black hole. Signals
can propagate in the traversable wormhole through two different channels. The
boundary channel is direct and instantaneous, while the bulk channel respects
the bulk causality and takes a certain amount of time to complete signaling. In
the latter case, we show that the signal frequency detected on the other side
is highly modulated in general. The time delay as well as the
frequency-modulation pattern could then be clear indications that the signal
comes out through the bulk channel. We discuss the characteristics of the
observed signal more explicitly for simple transitional configurations of the
black hole from/to the eternal traversable wormhole.
