Aside from obvious questions about the kinds of projects you'd be working on and the skills and technologies involved, I used to rely on a few simple tests:
1. What is the employee turnover like, and if people have left recently, why? [This is a reasonably objective proxy for what the rank and file staff really think, and they collectively know this potential employer much better than me.]
2. Did they ask brain teaser type questions or do a pop-psych personality test in the interview? [Do their hiring practices suck, in which case even if they hired me, would I want to work with anyone else they hired? Also tends to indicate an unhealthy assumption of superiority and desire to conduct one-sided interviews, which is a red flag in its own right.]
3. Will they show me a sample of their production code and of the related documentation, and if so, what is it like? [If they won't and don't have a very good reason, my assumption is that they aren't confident their code will impress, which is not a good sign. If they will, what I see is about as good a marker for whether they're any good technically as any benchmark you can fit into an interview.]
4. What does the car park look like on the way in? [Far from scientific, but a handful of executive cars in reserved spaces and then nothing but old models in the few remaining spaces is rarely a good sign.]
5. Does the contract impose on my life outside of work? For example, would I need permission from the employer to take on an unrelated side job that had no bearing on my ability to work properly for them? Would they be claiming IP rights to any work I did unrelated to that employment, so that for example I couldn't release a hobby project as Open Source if I wanted to? [Any attempt to control life completely outside work is a fairly reliable red flag for a generally over-controlling attitude and/or a show run by HR/legal rather than sensible management or the people I'd actually be working with.]