Get Parent ID(s) of record on Form

I'm "looking after" a CRM system which has been upgraded to CRM 2015 from CRM 2011.

There are a lot of forms and at one point, the user goes to a form and creates a record.  The users can get to this form from 1 of 2 ways.  On saving the form 1 of 2 relationship fields is populated.  I want to access this field in javascript before the form is saved to validate it.  

How do I do this?

Thanks