I will discuss how critical Casimir effects can be accessed through direct numerical simulation of a model binary fluid as it passes through the demixing transition in the high density phase. Simulations were performed in the semi grand canonical ensemble, in which the fluid composition fluctuates at fixed density. In slab geometry, a critical pressure anisotropy is observed in which the force per unit area parallel and perpendicular to the confinement direction are different. The measured anisotropy collapses into a universal scaling form closely related to that for the critical Casimir force. The critical Casimir force itself can be defined as the critical excess contribution to the generalized pressure function, P_perp - n *mu, conjugate variable to volume changes at fixed density n, with P_perp the pressure perpendicular to confinement and mu the chemical potential. In the context of existing experimental and numerical studies of the critical Casimir force, such an approach could open the way to dynamical and non-equilibrium studies.