[development] easy way to check if a value has changed (nodeapi)

Hi there, I'm working on a (node) profile system that has a checkbox to allow people to be informed by postal mail. If checked the system sends a mail with the details to a certain email address. Of course this email shouldn't be sent every time the profile is updated, but only when the checkbox was previously unchecked (if it was checked allready, nothing should happen). For the moment I'm thinking of creating a second (hidden) value to which I compare on 'update' and sync with the actual value after the check. But I'm thinking there must be an easier way, but can't find it in documentation or by inspecting $node on 'update'. Tnx, HnLn