Existence of params[:id]

ok, i'm sure I have forgotten or missing something, i hope you guys
can help:

if a user types :controller/edit/1, then he gets to edit the object
with the :id of 1.

If he only types in :controller/edit I want it to redirect him
somewhere else.

How do I test for the existence of params[:id] in an if statement. Or
is there a better way to accomplish this?