How do I find my respondent's contact info?

The other day, I received one of my lead forms back with no phone number and no email address, however they wanted me to contact them!  I digress….I found the potential customer but only because our business serves a specific area however, I was wondering...How would I do a “reverse look-up” of where the email came from? If that is not possible, I’d like to slip that in the suggestion box for instances just like this one.  Thank you and Im hoping someone can help me with this.  Have a blessed day