The main aim of our investigation is the search for new signs to predict solar flares. There are at least two ways to find such signs. The first one is statistical, where it is necessary to analyze a number of active region (AR) physical characteristics (area, full fluxes, spectra, etc.) and find correlations between some physical parameters and the flaring activity of the ARs. The other way is the deep analysis of ARs (specrum types in microwaves, polarization configuration of microwave sources, complexity of photospheric magnetic fields, etc.) to find the physical reasons of flares. The increased brightness of microwave radiation in 2011--2015 is considered as a preflare sign in this paper. There are a number of signs of preflare active region activity both in the optical and radio bands. An example of the preflare sign is the degree of complexity of photospheric magnetic fields in the AR. In microwaves, it is possible to estimate the radio emission flux from the AR or the brightness of radio emission sources at one or several frequencies. This paper analyzes the increased brightness of microwave sources in the 24th cycle of solar activity and the full microwave fluxes of ARs using the RATAN-600 data. It is shown that some ARs with high brightness temperatures generate a number of powerfull (M and X class) flares and also that the spectra of full microwave fluxes of the AR in the case of NOAA~12192 have features of high flare activity.