how to check if a user exist in database?

hi guys i use Profile provider.



now my problem is i wanna check if a user exists there or not.



for example someone may go to this url



http://localhost:4712/WebM/user.aspx?nick=hello



if user "hello" exist in database ok if not then redirect user to another page?




its because someone may paste something like this user.aspx?nick=aksdhadajkdhakjsdhakjsd



...