  We present an accurate numerical determination of the crossover from
classical to Ising-like critical behavior upon approach of the critical point
in three-dimensional systems. The possibility to vary the Ginzburg number in
our simulations allows us to cover the entire crossover region. We employ these
results to scrutinize several semi-phenomenological crossover scaling functions
that are widely used for the analysis of experimental results. In addition we
present strong evidence that the exponent relations do not hold between
effective exponents.
