A 32-year-old multigravida (G3P2) at 39 weeks is in active labor. Partograph shows cervical dilatation crossing the action line at 6 hours of admission, with adequate uterine contractions (4 contractions in 10 minutes, each lasting >40 seconds). Fetal heart rate is 140 bpm with good variability on CTG. What is the most appropriate next investigation to confirm diagnosis and guide management?