  Pushed by a number of advances, electromagnetic observatories have now
reached the horizon scale of supermassive black holes. The existence and
properties of horizons in our universe is one of the outstanding fundamental
issues that can now be addressed. Here we investigate the ability to
discriminate between black holes and compact, horizonless objects, focusing on
the lensing of hot spots around compact objects. We work in particular with
boson and Proca stars as central objects, and show that the absence of a
horizon gives rise to a characteristic feature -- photons that plough through
the central object and produce an extra image. This feature should be universal
for central objects made of matter weakly coupled to the standard model.
