First step: meet the guys, in their own environment, with at least another person, with ideally a different character than you, on your side. When one of you ask questions, the other should concentrate on listening and watching. Switch roles. After the meeting, you can exchange your impressions.
Very basic questions, for which you can be given easily official evidence that you can check, but this amounts to background check and audit of the firm:
- Is the company incorporated?
- Do the company has offices it rents with its own money?
- Has the company subscribed insurance policies, paid bills?
- Are they currently talking to other investors than you? (If they don't, it is not a good sign. Why are you so exceptional?)
- Did some of the founders or actual partners run something before? (If none did, this is a very different kind of investment, especially if you are not knowledgeable about the technological domain).
- What is at stake for actual owners and founders? Are they fully invested? Do they have other jobs or business running? Do they own real estate property, do they have their family, their partners in the area? If they have life partners or parents, do these have jobs, can you meet them?
- At least someone in the team should be able to tell you without looking or computing how much money they spend in a week, a month, or especially last month, what is their current balance. If the team has not at least someone obsessed by the accounting/business part of the management, they got bigger problems than finding new equity partners.