There are various equivalent definitions for a semisimple Lie algebra.  Cartan's criterion states that a Lie algebra is semisimple if its Killing form is non-degenerate.  This is the test applied by Query.

Query(Alg, "Semisimple") returns true if Alg is a semisimple Lie algebra and false otherwise.  If the algebra is unspecified, then Query is applied to the current algebra.

Query(S, "Semisimple") returns true if the subalgebra S is a semisimple Lie algebra and false otherwise.

The command Query is part of the DifferentialGeometry:-LieAlgebras package.  It can be used in the form Query(...) only after executing the commands with(DifferentialGeometry) and with(LieAlgebras), but can always be used by executing DifferentialGeometry:-LieAlgebras:-Query(...).