  We propose a novel method to examine whether Galactic double neutron star
binaries are formed in the LISA band. In our method, we assign an effective
time fraction $\tau$ to each double neutron star binary detected by LISA. This
fraction is given as a function of the observed orbital period and eccentricity
and should be uniformly distributed in the absence of in-band binary formation.
Applying statistical techniques such as the Kolmogorov-Smirnov test to the
actual list of $\tau$, we can inspect the signature of the in-band binary
formation. We discuss the prospects of this method, paying close attention to
the available sample number of Galactic double neutron star binaries around
1mHz.
