Sure, duplicate the mesh object container and reduce it's dimensions so it fits within the original. Next change it's attributes to positive. Create a cylinder or cube and make it's attributes negative. Use the negative object to cover the upper area of the smaller container and create the surface of the fluid. Make sure the original mesh is set to neutral and group the whole thing together. As long as the original container is not too complex in shape your "Boolean" mesh should now appear as filling the bottom of the new container. Retexture this object and the negative primitive as well to provide contrast and voila! Simple huh?