The fluid level is not a variable. One needs to use other available variable to detect the position of the free surface. A good indicator is a gradient of density or volume fraction. This gradient will have its maximum at the surface level. Therefore, define a line at the location of interest, parallel to the gravity vector (i.e. perpendicular to the fluid surface). To visualize the location of the surface, create a chart with the ordinate displaying Density.Gradient. Its maximum will be at the free surface.