Are your table scripts under source control? Most (all?) source control systems can be set to email certain people when file changes are checked in.

Do you have a formal development process which involves impact analysis and code review? Do you have a daily build or continuous integration process with smoke tests?

Do you create your test database from scratch as part of your build? If so, you could run a schema check script against the previous build.

Do you have coding standards that say 'Don't use select *'? Do you limit the use of nullable columns and force default values for new columns added?

There are lots of ways to catch what you want.