R: test what objects a function takes from the enclosing environment?

When defining an R functions, I sometimes miss that it relies on objects from the enclosing environment. Something like:

a <- 1
fn <- function(x) x + a

If this happens unintentionally, it can lead to problems which are difficult to debug.

Is there a simple way to test whether fn uses objects form the enclosing environment?

Something like:

test(fn=fn, args=list(x=1))
## --> uses 'a' from enclosing environment of 'fn'