The most important thing to look for is a scam. Not getting the money you want is one thing, but losing the money you have by falling for a scam is much worse. A lot of people pose as angel investors, venture capitalists, etc. The best way to find the fakes is to check them out.
Most VCs are listed in Pratt's Guide. Venture Capital firms don't generally advertise, because they don't have to. But fake venture capital firms do. Venture capital firms usually don't sign non-compete or non-disclosure forms --- again, because they don't have to. And they definitely don't ask for money. They already have it
Fake angel investors are much harder to spot. If they do not ask good questions, seem too eager to invest, or ask for "due dilligence" fees up front, they are probably scammers, although that is not necessariy true. Most importantly, you need to have a two-way dialog with an angel investor for it to work. If they are asking all the questions and you are not getting answers, then move on.