How to detect attribute changes from model?

You should look at ActiveModel::Dirty module:
You should be able to perform following actions on your Claim model:

claim.status_changed?  # returns true if 'status' attribute has changed
claim.status_was       # returns the previous value of 'status' attribute
claim.status_change    # => ['old value', 'new value'] returns the old and 
                       # new value for 'status' attribute

claim.name="Bob"
claim.changed # => ["name"]
claim.changes # => {"name" => ["Bill", "Bob"]}

Oh! the joys of Rails!