If/then for two different query string paramaters?

I have an odd situation.  I have a aspx page that I need to look at the incoming query string.  If FormID is passed, I need the page to execute one bit of code that I've written in my code behind.  If SurveyID is passed, then the page need to look at some other code I've written in the Code Behind. 
How do I tell my code behind to look for FormID and/or SurveyID?   I have the code written for both scinerios, just not sure how to get the page to determine which QueryString comes into the page. 
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