  Most of the properties of black holes can be mimicked by horizonless compact
objects such as gravastars and boson stars. We show that these ultra-compact
objects develop a strong ergoregion instability when rapidly spinning.
Instability timescales can be of the order of 0.1 seconds to 1 week for objects
with mass M=1-10^6 solar masses and angular momentum J> 0.4 M^2. This provides
a strong indication that ultra-compact objects with large rotation are black
holes. Explosive events due to ergoregion instability have a well-defined
gravitational-wave signature. These events could be detected by next-generation
gravitational-wave detectors such as Advanced LIGO or LISA.
