thread_variables

Returns an array of the names of the thread-local variables (as Symbols).

thr = Thread.new do
  Thread.current.thread_variable_set(:cat, 'meow')
  Thread.current.thread_variable_set("dog", 'woof')
end
thr.join               # => #<Thread:0x401b3f10 dead>
thr.thread_variables   # => [:dog, :cat]

Note that these are not fiber local variables. Please see Thread#thread_variable_get for more details.