The critical behavior of fluids is analyzed in terms of an effective Hamiltonian which is obtained by a Hubbard-Schofield transformation that maps the fluid Hamiltonian onto a Wilson-Fisher Ising-type Hamiltonian. Within this approach we find an approximate relation between critical temperature and critical density, depending on the parameters of the interaction potential, and compare it with simulation results. In the framework of the effective Hamiltonian we examine the critical data for the square-well fluid obtained using thermodynamic-scaling Monte Carlo and discuss the impact of finite-size effects on the Monte Carlo studies of the fluid criticality.