R get the environment created by a function at the call

I would like to get the environment created by a function when it is runned WITHOUT modifying the function source (ie from outside of the function), is it possible ?

fn=function()
{#Here a new environment is created at each call, how to get it ?
 #This environment can be access with environment() but only (to what I know) 
 #from inside the function

  ...
}

I would like something like this:

env=some_function(fn())

where env is the environment id created by fn at the call.