How to find the structure requires in wrintg SAPscript?

hi all gurus,

I've some doubts regarding the sapscripts.I understand the usage of perform which is calling a subroutines from abap program.my doubt here is whenever i writing the sapscript,i need to pass the value..for example &cust_name$ to the subroutines inorder for it to search for..let say customer ID. how should i know tat the &cust_name& is actually from which structures?

Thank You.