IsEmpty Variable Testing Function
Before you can do anything with a variable, it must exist. This function helps ensure that session variables and the like have been initialized before you try to use them.
If you had a variable called LogonID, your syntax would be:
NoLogon = IsEmpty(LogonID)
if NoLogon = TRUE then
response.write "you must log on before you view this page."
end if
ASP Variable Testing Functions