Unstable meniscus flow leads to slab surface defects during continuous casting of steel, due to level fluctuations and vortex formation, which causes entrapment of argon bubbles and mold flux. Applying electromagnetic fields across the liquid steel pool, such as the "double-ruler" or "FC-mold" braking system, has been commercialized to stabilize meniscus flow. Plant measurements were performed using nail boards to quantify meniscus flow in a typical steel slab-casting mold with a slide gate system. The shape of the meniscus level, the surface velocity, and the direction of meniscus flow, are all quantified with time and location by analyzing the shape of the skull of solidified steel that encases each dipped nail. The results reveal interesting insights into time-variations of the flow pattern, which cannot and should not be detected with a standard mold-level sensor used for flow control. Further, the effect of applying the electromagnetic braking field on the flow pattern is revealed.