I use various generic app icon images that I found on the web that seem to be either from sites offering this images for free or from sites that have themselves fround the icon images for free. Will Apple dig deep into whether or not you are submitting an app with legal usage of generic app icons (e.g. info icon, schedule icon, etc)?
Using app icon images legally
You never know.
That's why you must have documentation proving that you have the right to use the image.
How do I go about gaining documentation to prove that I have the right to use the image? Here are examples of where I am getting the images from:
1. Info icon (http://png-1.findicons.com/files/icons/2315/default_icon/258/info.png)
2. Calendar icon (http://simpleicon.com/wp-content/uploads/Calendar-1.png)
3. People icon (http://cdn.flaticon.com/png/256/27825.png)
4. Map icon (http://simpleicon.com/wp-content/uploads/map-7.png)
5. Handshake icon (http://esscoagm.macengsociety.ca/wp-content/uploads/2013/07/icon.png)
6. Book icon (http://eastersundayservice.com/wp-content/uploads/2014/03/Open_Book.png)
Probably not -- they apparently don't have time to do any kind of copyright/trademark infringement research on the apps they review. They would probably only flag something that is REALLY blantant (like using a Coke logo as your icon). But as the other guy said, ALWAYS follow the rules and don't steal anything.
You should find the contact info at each of those websites and ask for permission to use them.
Although the simpleicon web site appears to offer royalty free icons.