Looking about a customer from URL

Hi. I'm Learning about the possible conversation beetween CRM and olther client system. I'm working with Contact entity. I'd implement a Jscript (or ASP code) that check if exists a record about a Contact(checked by Full Name and Customers Number). If the record doesn't exists the system will open a CRM form into an Iframe with the pre-compiled data. If the record exists i would open his contact page. At the moment I understand how auto compile a form and open a particular form with an URL, for example: main.aspx?etn=contact&pagetype=entityrecord&extraqs=formid%3De7c97c17-2727-44b5-9416-ssqw322 How can i do a query search with a GET HTTP Call that implements this workflow? Thank you.