Very little. At the angel/seed stage, they're investing in the founders, so there's no expectation of patents, etc... They might check that you're incorporated in good standing, and ensure you have a solid startup/corporate lawyer, and have good employment and IP ownership agreements with your s...
Hell no. I've been in the startup world since 1995, and no one has ever asked me for such a thing.
The only due diligence a good angel will do are any of the following: 1) Ask anyone who they know that also knows you what they think of you; 2) Use your product to whatever extent possible. 3) Look at what others have invested in that may be competitive. 4) Talk with others who have failed with ...
This is a question that requires far more than a simple 10-minute answer, as due diligence is an extremely complex subject and only having a few "quick tips" would put you in a very vulnerable position. Generally speaking though, the key areas that you would want to focus on (depending on the ty...