  We present a series of test beds for numerical codes designed to find
apparent horizons. We consider three apparent horizon finders that use
different numerical methods: one of them in axisymmetry, and two fully
three-dimensional. We concentrate first on a toy model that has a simple
horizon structure, and then go on to study single and multiple black hole data
sets. We use our finders to look for apparent horizons in Brill wave initial
data where we discover that some results published previously are not correct.
For pure wave and multiple black hole spacetimes, we apply our finders to
survey parameter space, mapping out properties of interesting data sets for
future evolutions.
