In FileMaker 10 and up you can use an OnObjectExit script trigger to perform a script that checks.
In previous versions ( and if you want in newer versions as well ) you could use Field validation
First, thank you for your answer. It is probably just want the novice programmer needed. I looked up OnObjectExit and found it in the help menu - the only thing I didn't see how to actually do it.
Is there an example somewhere that you know of?
I would check out the phone example thatis on their website. There used to be a video on the various script triggers for 10 but it seems as though they are now gone from the site.