In the GLAS altimetry products is there an indicator to the presence of clouds?

Yes, it is an element of: GLA06 (GLAH06), GLA12 (GLAH12), GLA13 (GLAH13), GLA14 (GLAH14), GLA15 (GLAH15)

The Multiple Scattering Warning Flag, i_cld1_mswf (cld1_mswf_flg) is based on the total column optical depth (aerosol plus cloud) calculated in GLA11 using 532nm. It is intended as a way to quickly obtain information about the potential severity of multiple scattering with regards to the range-to-surface calculated by the altimetry processing software.The multiple scattering warning flag will have values ranging from 0-14, based on the total column optical depth. A warning flag value of 15 will signify 'invalid?'. An invalid will be encoded if an optical depth in any of the layers in the 1-second column could not be calculated. This usually occurs in a very optically 'thick' cloud which extinguishes the signal. It could also occur if the extinction-to-backscatter ratio assignment is set too high, causing the transmission calculations in the lidar inversion to go out-of-range.

The Medium Resolution Cloud Availability Flag, i_MRC_af (MRC_af_flg) tells how many cloud layers were found at this resolution from the 532 nm channel.

The Full Resolution 1064 Cloud Top, i_FRir_cldtop (d_FRir_cldtop) parameter is the full resolution (40 Hz) cloud top height obtained from the 1064 atmospheric channel. This parameter is for a 1 second record.