Due to the unlicensed spectrum nature of the optical wireless communication, for a transceiver pair, the receiver needs to periodically sense the communication media, aiming to better schedule its transmission. This work proposes two detection rules on whether there are optical signals falling into the receiver field of view within certain time slot(s), based on the sum number of received photons over all sensing slots as well as the number of received photons in each slot. Considering that the data symbols are unknown, we adopt generalized likelihood ratio test (GLRT), which is known to be optimal for detection with unknown parameters based on finite samples. We also provide asymptotic results on the miss detection probability.