I don't really have an answer for you, this is more of an add on to your question.
I can't tell if my 5 1/2 month old is really hungry at night either. Just like you, I know I should be able to tell by the way she cries, but I can't tell. She's nursed in the middle of the night everytime she's woken since she was born, so obviously, she wants to nurse, but is it for comfort or hunger? Who knows.
I've thought about trying PU/PD, but I'm overwhelmed and afraid of trying (afraid of the crying).