When evaluating skim coat, prioritize Adhesion Strength (measured in MPa) to ensure it bonds permanently to the substrate. For interior walls, a compressive strength of ≥3.0 MPa is standard, while exterior grades require higher resistance. Additionally, check the Water Retention Rate (should be >90%); high water retention prevents the mix from drying too fast, reducing the risk of cracking and powdering. Lastly, verify the fineness of the powder, typically measured by mesh size, where a higher mesh (e.g., 325 mesh) results in a smoother, 'mirror-like' finish.