The detection and number estimates of supermassive binary black holes (SMBBHs) provide important constraints on the hierarchical models for galaxy formation and evolution. I will present two different approaches for the possible identification of SMBBHs. 1.Radio-optical studies of X-shaped radio sources:X-shaped radio galaxies are extragalactic radio sources that present two pairs of radio lobes passing symmetrically through the center of the host galaxy, giving the galaxy the X-shaped morphology seen on radio maps.This morphology can reflect either a recent merger of two supermassive black holes or the presence of a second active black hole in the galactic nucleus. This scenario is studied by determining the mass, luminosity, jet dynamic age and starburst of a sample of X-shaped sources and comparing the results to a sample of radio-loud active nuclei with similar redshift and luminosities. 2.Compact radio emission in ULX objects: Ultraluminous X-ray sources (ULXs) have luminosities exceeding 10E39 erg/s, suggesting either the presence of black holes larger than stellar mass black holes or sources apparently radiating above the Eddington limit. I will present milliarcsecond-scale radio observations of some ULXs located within optical bright galaxies, resolving their compact radio emission, and measuring its brightness temperature and spectral properties. This allows us to uncover the nature of these sources and investigate whether they are intermediate mass black holes or supermassive black holes stripped of their accretion disks in post-merger systems.