In addition to what you can find in the upgrade manual Oracle E-Business Suite Upgrade Guide the list of sanity check question would include something like the following:
- Create/Update users/responsibilities/functions/menus
- Submit different types of concurrent programs
- Ensure CM and Workflow is working as expected
- Verify OPMN components and log files
- Verify your custom tops (if any exists)
You should come up with your own list when you upgrade your test instance and verify the same for production upgrade.
You may Google "R12 Upgrade Lesson Learned" and/or "R12 Post Technical Upgrade" and you should get many hits.
Hussein's post covers it all, but just to add on to what was said, Since we dont really know your advance configuration such as for example it may be SSO, SSL, Shared Appl top, RAC, DG and etc, you will have to yourself make a list of what has to be checked based on your existing configuration.
If it is a simple EBS configuration on which you have done the upgrade, then the list posted by Hussein would suffice the need.