Something like this to check is user is a member of the Quality Center site(depends on the language you use of course):
Dim cust As Customization
Dim uList As CustomizationUsers
Set cust = TDConnection.Customization
Set uList = cust.Users
If Not uList.UserExistsInSite("<usernamecheckedtobechecked>") Then
msgbox "User does not exist in site"
End If
----
For checking if user is a member of a project, you need different code.
Dim cust As Customization
Dim uList As CustomizationUsers
Set cust = TDConnection.Customization
Set uList = cust.Users
If Not uList.UserExistsInSite("<usernamecheckedtobechecked>") Then
msgbox "User does not exist in site"
End If
----
For checking if user is a member of a project, you need different code.