@Curmudgeon, I'll give you a few answers from my experience, but understand I have not invested in any deals under the new, looser, subscription rules. Only the old private placement rules: (
https://www.sec.gov/oiea/investor-alerts-bulletins/ib_privateplacements.html)
RE "what to look for" my approach has always been to invest in people. To do this I need to know the people personally, maybe after meeting and talking with them in connection with the deal. Ideally, we have mutual friends. The one time I didn't do this I relied on the reputation of a local restaurant group who had a string of very successful restaurants. I never did meet any of them directly. That was the $50K that went down the toilet.
After being satisfied with the sellers, I do look at the business proposal. It has to be a business I understand and the thinking has to make sense to me. But the sellers are the experts, they are putting their blood and treasure into the deal, so if they don't know more than I do, why am I here? (If they are not putting blood and treasure into the deal, I am already gone.)
My wife and I were pitched by a professional friend of hers who wanted to start a bank. It was very convincing but I declined because I don't know how to read bank financial statements and I really don't understand the risks in the business. He went on to do very well and I think we would have profited but I am completely comfortable with sticking to my decision to only buy things I understand.
Re what are you buying, in my case it was always a minority share of stock. In the deals I saw, the company founders always kept enough to have controlling interest. So it went back to trust, because the majority can screw the minority shareholders pretty badly if they choose to. There are state laws that protect us, but those laws and $20K in legal fees are probably the minimum ticket for redress.
Re funding sites like @Onward mentions, the only sure thing about what is offered there is that the sellers have been unable to get the deal funded locally. Probably because it is a stinky deal, maybe because they are inept or unqualified in the financial aspects of what they are doing. Viewed as lottery tickets, I think the sites could be a lot of fun. Viewed as an investment, not so much. I never buy lottery tickets, so I won't be following @Onward's path, but I think he/she has made an accurate assessment of the risks and hope he/she enjoys the rides.