Working with Request.QueryString?

I have some questions regarding Request.QueryString for this scenarios:

default.aspx
default.aspx?Page=1
default.aspx?Page=1&ID=200
default.aspx?Page=1&ID=

How can I get if there is Page or ID in QueryString or ID has some value or ...?

I now I can count how many QueryStrings are there? But I don't know how can I get if there is Page or ID in the link (default.aspx)?Object obj = Request.Params["key"]
or Request.Form["key"]
QueryString is an object of NameValueCollection for list of all members you may navigate toMSDN