test = SomeObject.new
test.methods - test.class.superclass.new.methods

Or is there a shorter, less convoluted way?

Update November 12th: yes there seems to be…

puts String.instance_methods(false)

From the Ruby For Rails Book, page 254, paragraph 9.8.1….

But: String.instance_methods(false).size = 83 , whereas (test.methods - test.class.superclass.new.methods).size = 102 (with test a string object).

>> puts (test.methods - test.class.superclass.new.methods - String.instance_methods(false)).sort
<
<=
>
>=
all?
any?
between?
collect
detect
each_with_index
entries
find
find_all
grep
inject
map
max
member?
min
partition
reject
select
sort
sort_by
zip
=> nil
>> exit

The difference seems to be the methods of the mixed in modules…