Find path of an executable in vim

On some machines I'm using an external executable to format text (it's named 'par'). Therefore I'd like to detect if the executable is installed.

Edit: I just want to detect if the executable is available in my .vimrc itself. So I prefer internal functions over external utils like 'which'. Prince Goulash's solution is already very helpful and I'm going with it, despite I'd prefer finding the exact path. Thanks.