All you have to do is go to tool==>expression=> and under listed expressions change it to unused expressions.. This will show you all unused expressions.
Hi HariharanB: I use the UF_MODL_ask_features_of_exp() to retrieves the unused exps and the UF_SKET_ask_exps_of_sketch() to retrieve the used exps in display part, also need to identifying some other exps.