Hello,
I'm using Hans' macro suggested some time before.
I've just updated Ctx MkIV and my sample code:

----
\starttext
  \def\MyMacro{Ahoj}

  \def\MyMacWithPars#1#2{Something}

  \def\Undefined{}
  \let\Undefined\undefined

  \def\DoNothing{}
  \let\DoNothing\donothing

  \startluacode
    if CONTEXTLMTXMODE == 0 then
        local d = tokens.defined
        local c = tokens.create

        function tokens.defined(s,b)
            if b then
                return d(s)
            else
                return c(s).cmd_name == "undefined_cmd"
            end
        end
    end

    --

    local defined = tokens.defined

    for _, v in ipairs{ "MyMacro", "MyMacWithPars", "Undefined", "DoNothing", "NonExisting" } do
      print(string.format("\letterpercent-12s", v), defined(v), defined(v, true), defined(v, false))
    end
  \stopluacode

  Aaa
\stoptext
----

gives:
"
MyMacro         false   false   false
MyMacWithPars   false   false   false
Undefined       false   false   false
DoNothing       false   false   false
NonExisting     false   false   false
"

IOW, I'm not able to determine whether a control sequence has not been defined (or has \undefined value) or not.

Am I doing a trivial mistake or has anything changed?

Thank you for patience...

Lukas