[support] Setting up a content check rule

I had an instance just this weekend where a contributor made an entry in an event system and forgot to enter some data. I find I need to validate data in an event entry form and check if fields are not empty. I do not need to check at this point if data is correct. Can someone on list guide me on how to set up the <Rules> <Conditions> to check each field, particularly the date field. Alternatively is there another way to check/validate fields and advise the contributor. Can someone point me in the right direction please thanks Roger