We don't exactly see them directly growing, but the strong gravitational effects a blackhole exerts on the stars/blackholes around it gives us a pretty good ide

We don't exactly see them directly growing, but the strong gravitational effects a blackhole exerts on the stars/blackholes around it gives us a pretty good idea of the size of the blackhole, among other things. We can also infer the size of a blackhole through gravitational waves produced due to it/around it, which is the subject of this work.