Rails: How can I know the type of the argument a method expects to?

I'm programming in Ruby on Rails. How can I know the type of the argument a method expects to?

for example, lets say I have a method:

def foo(bar)
    @bar = bar
    @baz = get_user(@bar.user)
end

How can I know which object should I send to bar?

p.s: I am trying to understand a code not written by me