Ok we have a production erlang system up and running, but is is leaking memory... wait Erlang doesn't leak memory but processes. There is a talk about it from Berard Dugga and there is one slide that tells it all. I copied the slide here as a note to myself where to start looking.
Number of process
length(erlang:processes())
Queue length of (esp. for busy processes)
erlang:process_info(Pid,message_queue_len)
Total memory use
erlang:memory()
Another usefull tool is Erlang TOP (etop) that shows a lot of information.