  Monte Carlo simulations within the grand canonical ensemble are used to
obtain the joint distribution of density and energy fluctuations $p_L(\rho,u)$
for two model fluids: a decorated lattice gas and a polymer system. In the near
critical region the form of $p_L(\rho,u)$ is analysed using a mixed field
finite-size-scaling theory that takes account of liquid-vapour asymmetry. Field
mixing transformations are performed that map $p_L(\rho,u)$ onto the joint
distribution of critical scaling operators \ptMEstar\ appropriate to the Ising
fixed point. Carrying out this procedure permits a very accurate determination
of the critical point parameters. By forming various projections of \ptMEstar ,
the full universal finite-size spectrum of the critical density and energy
distributions of fluids is also obtained. In the sub-critical coexistence
region, an examination is made of the influence of field mixing on the
asymmetry of the density distribution.
