Golden Rule 1 : Single-Span is Inherently Smooth
A single span curve or surface is always smooth, whereas multi-span can have a continuity break between the spans.
This can be seen by applying a curvature comb using Evaluate → Curve Curvature.
A 'continuity break' is any deviation from a smooth curvature comb plot. To be more precise, you need to understand what continuity level you are aiming for. More technical information on this can be found in the Evaluation1: Curve Continuity Theory Builder.
In practice, we use our visual judgement to notice a 'smooth' or 'lumpy' curvature plot.