how do you keep up to date on gems ?

I was asked an interview question on how I would keep an app's gems up to
date. Suppose there was a new gem because of some security issue for
instance ?

Where I worked before, we locked the gems in the gem file with a
particular version with the notation of something like:
gem 'multi_json' , '~> 1.8.2'

That way we would not get surprises when we updated the gems as changes
could occur without our being able to know what they where and the app
would be unstable. I've also found when some one gives you an app to work
on and it has not been used for several months, if the gemfile has no
versions on the gems then you will have a hard time figuring out why
everything is broken or what gem version it used to work in.

I did not find there to be an easy answer to this question given that a
gem file can contain many gems and knowing when to go to a new version is
not clear. At a certain point in time between projects, we might try to
update the gems. Is there a simple way to tell how far out of date the gems
in the gemfile are using a command of some kind ?