is_debug_mode <- function()
{
  exists(".DEBUG", envir = globalenv()) &&
    get(".DEBUG", envir = globalenv())
}

set_debug_mode <- function(on = FALSE)
{
  old_value <- is.debug.mode()
  .DEBUG <<- on
  invisible(old_value)
}

if(is_debug_mode())
{
  #do some logging or whatever
}

set_debug_mode(TRUE)   #turn debug mode on
set_debug_mode(FALSE)  #turn debug mode off