Well, getting back to your original question ... from your NT4 logon scripts could you check against the @DOMAIN and @LDOMAIN macros, or since all your machines are NT5 - could look at Howards ADSI UDF's for:

GetComputerOU()

-and-

GetUserOU()

Not sure what these functions would return when not joined to NT4, but having them returning nothing versus something would prolly tell you something.