During some development work regarding HEVC I encountered a camera that streams its pictures in multiple NAL units. When analyzing the bitstream I noticed that there are 3 NAL units per picture. I noticed this with the first_slice_segment_in_pic_flag = 1 for the first NAL unit and first_slice_segment_in_pic_flag = 0 for the 2 following NAL units.
Is there any way of knowing how many NAL units make up a full picture or which NAL unit is the final one of the current picture without looking at the 4th NAL unit (the first one of the next picture)?
No its not possible. You get to know end of the picture after decoding last CTU of the picture or when you get the first slice of the next picture.