  We present second epoch optical spectra for 30 changing-look (CL) candidates
found by searching for Type-1 optical variability in a sample of active
galactic nuclei (AGNs) spectroscopically classified as Type 2. We use a
random-forest-based light curve classifier and spectroscopic follow-up,
confirming 50 per cent of candidates as turning-on CLs. In order to improve
this selection method and to better understand the nature of the not-confirmed
CL candidates, we perform a multi-wavelength variability analysis including
optical, mid-infrared (MIR) and X-ray data, and compare the results from the
confirmed and not-confirmed CLs identified in this work. We find that most of
the not-confirmed CLs are consistent with weak Type 1s dominated by host-galaxy
contributions, showing weaker optical and MIR variability. On the contrary, the
confirmed CLs present stronger optical fluctuations and experience a long (from
five to ten years) increase in their MIR fluxes and the colour W1-W2 over time.
In the 0.2-2.3 keV band, at least four out of 11 CLs with available SRG/eROSITA
detections have increased their flux in comparison with archival upper limits.
These common features allow us to select the most promising CLs from our list
of candidates, leading to nine sources with similar multi-wavelength
photometric properties to our CL sample. The use of machine learning algorithms
with optical and MIR light curves will be very useful to identify CLs in future
large-scale surveys.
