  In the third catalog of active galactic nuclei detected by the $Fermi$ Large
Area Telescope Clean (3LAC) sample, there are 402 blazars candidates of
uncertain type (BCU). The proposed analysis will help to evaluate the potential
optical classification flat spectrum radio quasars (FSRQs) versus BL Lacertae
(BL Lacs) objects of BCUs, which can help to understand which is the most
elusive class of blazar hidden in the Fermi sample. By studying the 3LAC
sample, we found some critical values of $\gamma$-ray photon spectral index
($\Gamma_{\rm ph}$), variability index (VI) and radio flux (${\rm F_R}$) of the
sources separate known FSRQs and BL Lac objects. We further utilize those
values to defined an empirical "high-confidence" candidate zone that can be
used to classify the BCUs. Within such a zone ($\Gamma_{\rm ph}<2.187$,
log${\rm F_R}<2.258$ and ${ \rm logVI <1.702}$), we found that 120 BCUs can be
classified BL Lac candidates with a higher degree of confidence (with a
misjudged rate $<1\%$). Our results suggest that an empirical "high confidence"
diagnosis is possible to distinguish the BL Lacs from the Fermi observations
based on only on the direct observational data of $\Gamma_{\rm ph}$, VI and
${\rm F_R}$.
