best practice to indicate exception/validation error?

Hi all,

Just getting familiar with rails and am performing some checks in a
method within a controller. If the checks fail, I would like to
indicate to the user a custom error message and stop the update (the
method is updating a record in the model).

What are best practices to do this? I understand that for form
validation I should be putting validation checks in the model however
this is different in that the check is not against any one field in
the model. Should I be raising an exception?

Ideally, I would like to indicate the error to the user in a similar
fashion as when a form validation fails.