Dead simple: Get the method object and ask for its owner:

"foo".method(:upcase)
=> #<Method: String#upcase> 
"foo".method(:upcase).owner
=> String

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Ruby 1.9 adds a method Method#source_location that returns file and line number where that method is defined.

class Example; def method() end end
=> nil
Example.new.method(:method).source_location
=> ["(irb)", 11] 

"foo".method(:upcase).source_location
=> nil # String#upcase is a native method that's defined in C

Rumor has it in Ruby 1.8 there is a gem ruby18_source_location that does something similar.

Also see

• Using Ruby's Method objects for inspecting methods
• How to examine an unknown Ruby object