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 staff and contractors, and that's about it.
Varies greatly due to company focus and product(s) and/or services contemplated, but diligence focus at this level is usually focused on product viability and market opportunity, competitive landscape, team and path to market.
They waill check and verify all the facts and figures that were produced for securing the funds in pitch deck and financial projections.