One tip I found was if you Ctrl click on any folder you can attach a 'folder action' to it. There's a simple 'add - new item alert.scpt' action which alerts you to items being added to the folder. Not totally what I was looking for.
There's also a program called
File Monitor which allows you to monitor more than one folder at a time but again, the alert system is very simplistic. And doesn't tell you what file changed and it doesn't monitor any sub-folders within a folder without individually selecting them. Ideally there would be some kind of logging method, as well.