Hi KP, good question. Ideally, if I manage to get the desired time with the team, I ask the Founders to complete a set of Product (and sometimes high-level Tech) questions that is in essence an audit designed to uncover any red flags / elicit the desired qualities I mention. Answers to these questions then provide the basis for a deeper dive where required, such as sessions focused on being walked through their roadmap, etc..

I should also add that the deliverable from this first-run Due Diligence is essentially a document that either recommends or not the next level of DD (and hence a commitment of additional funds / time). This has proved particularly useful for clients not based in the Bay Area (I have quite a few Australian clients as an example).