\def\firstoftwo#1#2{#1}
\def\secondoftwo#1#2{#2}
\protected\def\ifscanable#1{
  \begingroup
  \edef\resrvd{%
    \def\noexpand\resrvd####1\detokenize{macro}:####2->####3&{%
      ####1\def\string\resrvd####2{####3}}%
    \edef\noexpand\resrvd{\noexpand\resrvd\meaning#1&}}%
  \resrvd
  \scantokens\expandafter{\resrvd}%
  \expandafter\endgroup\ifx#1\resrvd
    \expandafter\firstoftwo
  \else
    \expandafter\secondoftwo
  \fi}
test returns 1: \ifscanable\firstoftwo{1}{2}
\bye