That would be in the output (picture) settings in the Blu-Ray's menu. If you press the INFO button on the remote, it will usually tell you what the incoming resolution and Hz rating is. If it's properly set, the rating will be 24.
I have purchased a new Panasonic Blu ray player (BD60) capable of 24p output via HDMI. Does anyone know how to enable this capability on the my Samsung LN52B610 LCD TV?