The name of the completion function for the command foo is $_comps[foo].
To see the code of a function myfunc, run echo -E $functions[myfunc], or just echo $functions[myfunc] if you have the bsd_echo option on, or print -rl $functions[myfunc]. So to see the code of the completion function for the command foo, run echo -E $functions[$_comps[foo]]. Alternatively, run which $_comps[foo] if the function name has no alias.
This shows the code without comments (and with normalized whitespace: it's a human-readable dump of the bytecode that zsh stores internally). If you want to see the original file defined in the code, run **[whence -v $_comps[foo]](http://zsh.sourceforge.net/Doc/Release/Shell-Buil