1. I make sure the job description is inline with what I want to do. I also make sure it's documented in case 2 months into your new job your boss decides you need to focus on something else and you end up pigeon holed into something you don't want to do.
2. Make sure there is a solid career path that could help you grow within the organization. Ask the employer to highlight such a path for you.
3. Compensation, benefits, stock options, bonuses, etc. Make sure this is all clearly documented in the offer letter. If they say no to documenting it they will most likely end up not giving it to you once you are on board.
4. Make sure you clearly understand the company's strategy, growth plan and financials. You don't want to get into a place that will end up on it face a year later. Many times it's easy to spot such companies with simple research. If their strategy doesn't agree with you, you will end up frustrated very quickly.
5. I usually also do research on the products being sold and try to understand how I can personally add value to make these products more successful. This helps me determine if I will enjoy working on them or if I would be bored. Companies strategy might be good but if the current products are blah you'll be bored quickly and might not have the patience to wait for the strategy to unfold.