We present synthetic observations of star-forming interstellar medium structures obtained by hydrodynamic calculations of a turbulent box under the influence of an ionizing radiation field. The morphological appearance of the pillar-like structures in optical emission lines is found to be very similar to observations of nearby star-forming regions. We calculate line profiles as a function of position along the pillars for collisionally excited [O iii] λ5007, [N ii] λ6584 and [S ii] λ6717, which show typical full width at half-maximum of 2–4 km s−1. Spatially resolved emission-line diagnostic diagrams are also presented which show values in general agreement with observations of similar regions. The diagrams, however, also highlight significant spatial variations in the line ratios, including values that would be classically interpreted as shocked regions based on 1D photoionization calculations. These values tend to be instead the result of lines of sight intersecting which intersect for large portions of their lengths the ionized-to-neutral transition regions in the gas. We caution therefore against a straightforward application of classical diagnostic diagrams and 1D photoionization calculations to spatially resolved observations of complex 3D star-forming regions.