  The majority of brown dwarfs show some level of photometric or
spectro-photometric variability in different wavelength ranges. This
variability allow us to trace the 3D atmospheric structures of variable brown
dwarfs and directly-imaged exoplanets with radiative-transfer models and
mapping codes. Nevertheless, to date, we do not have an informed method to
pre-select the brown dwarfs that might show a higher variability amplitude for
a thorough variability study. In this work, we designed and tested
near-infrared spectral indices to pre-select the most likely variable mid- and
late-T dwarfs, which overlap in effective temperatures with directly-imaged
exoplanets. We used time-resolved near-infrared Hubble Space Telescope Wide
Field Camera 3 spectra of a T6.5 dwarf, 2MASS J22282889--431026, to design our
novel spectral indices. We tested these spectral indices on 26 T5.5--T7.5
near-infrared SpeX/IRTF spectra, and we provided eight new mid- and late-T
variable candidates. We estimated the variability fraction of our sample in
$38^{+4}_{-30}$%, which agrees with the variability fractions provided by
Metchev, et. al, (2015) for mid-to late-T dwarfs. In addition, two of the three
previously known variables in our SpeX spectra sample are flagged as variable
candidates by our indices. Similarly, all seven known non-variable in our
sample are flagged as non-variable objects by our indices. These results
suggest that our spectral indices might be used to find variable mid- and
late-T brown dwarf variables. These indices may be crucial in the future to
select cool directly-imaged exoplanets for variability studies.
