  Alon and Shapira proved that every monotone class (closed under taking
subgraphs) of undirected graphs is strongly testable, that is, under the
promise that a given graph is either in the class or $\varepsilon$-far from it,
there is a test using a constant number of samples (depending on $\varepsilon$
only) that rejects every graph not in the class with probability at least one
half, and always accepts a graph in the class. However, their bound on the
number of samples is quite large, since they heavily rely on Szemer\'edi's
regularity lemma. We study the case of posets and show that every monotone
class of posets is easily testable, that is, a polynomial number of samples is
sufficient. We achieve this via proving a polynomial removal lemma for posets.
  We give a simple classification: for every monotone class of posets there is
an $h$ such that the class is indistinguishable (every large enough poset in
one class is $\varepsilon$-close to a poset in the other class) from the class
of posets free of the chain $C_h$. This allows to test every monotone class of
posets using $O(\varepsilon^{-1})$ samples. The test has two-sided error, but
it is almost complete: the probability to refute a poset in the class is
polynomially small in the size of the poset.
  The analogous results hold for comparability graphs, too.
