The OS gem allows for some easy telling if you're on windows or not.

require 'os'
>> OS.windows? 
=> true

>> OS.bits
=> 32

>> OS.java?
=> true

>> OS.ruby_bin
=> "c:\ruby18\bin\ruby.exe" # or "/usr/local/bin/ruby" or what not

>> OS.posix?
=> false

>> OS.mac?
=> false

If there are any other features you'd like, let me know.

github.com/rdp/os

Related:

rubygems:
Gem::Platform.local
Gem.ruby

facets gem (similar)
require 'facets/platform'
Platform.local

the Platform gem