The command to see what gems have more recent versions and can be updated is bundle outdated. This will show you all gems that could be upgraded, but unlike bundle update, will not actually update your gems (changing Gemfile.lock and installing new gems.)
It will output something like:
$ bundle outdated
Updating git://github.com/panozzaj/andand.git
Fetching gem metadata from http://rubygems.org/.......
Fetching gem metadata from http://rubygems.org/..
Outdated gems included in the bundle:
* ZenTest (4.6.2 > 4.5.0)
* activesupport (3.1.1 > 3.0.9)
* builder (3.0.0 > 2.1.2)
* i18n (0.6.0 > 0.5.0)
* activemodel (3.1.1 > 3.0.9)
* erubis (2.7.0 > 2.6.6)
* rack (1.3.4 > 1.2.4)
* rack-mount (0.8.3 > 0.6.14)
* rack-test (0.6.1 > 0.5.7)
* actionpack (3.1.1 > 3.0.9)
* mail (2.3.0 > 2.2.19)
* actionmailer (3.1.1 > 3.0.9)
* arel (2.2.1 > 2.0.10)
* activerecord (3.1.1 > 3.0.9)
* activeresource (3.1.1 > 3.0.9)
* devise (1.4.2 2a5ad46 > 1.4.2 e76ba05)
* railties (3.1.1 > 3.0.9)
* rails (3.1.1 > 3.0.9)
* sequel (3.28.0 > 3.20.0)
* sinatra (1.3.1 > 1.0)
To break it down, “sinatra (1.3.1 > 1.0)” means that I have version 1.0 and version 1.3.1 is available.
Potential stumbling blocks
Getting a ‘Could not find task “outdated”.’ error message when trying to run bundle outdated?
At the time of this writing you need to have the right version of Bundler installed with gem install bundler --pre. This installs Bundler version 1.1.rc, which should also be much faster than earlier versions of Bundler. I would expect this portion of this post to be outdated in the near future.