Discern post-event in music function

[Simon Albrecht]

Hello,

I currently use this implementation of the \after function:

after =
#(define-music-function (t e m) (ly:duration? ly:music? ly:music?)
   #{
     \context Bottom <<
       #m
       { \skip $t <> -\tweak extra-spacing-width #empty-interval $e }
     >>
   #})

Unfortunately, it doesn’t allow using it with e.g. \tempo expressions, 
since it expects a post-event. So I need the music function to do 
different things depending on whether the second argument is a 
post-event or not.  How could one implement a ‘post-event?’ predicate?

Best, Simon