Several ways (overkill answer, but leaving all options I know because why not):

You can try evaluating this in the scratch buffer

    (if (member 'nativecomp features)
        (message "yay")
      (message "nay"))

Or check if a non-interactive function called `native-compile` exists.

Or open help for an elisp function (next-line, for example):

    c-h f next-line RET

Gives me:

    next-line is an interactive native compiled Lisp function in ‘simple.el’.

Or, m-x disassemble next-line RET