I don't see many scams that have someone posing as an investor at their core. In the US, you can check if they're a qualified investor and that will remove those that are posing.
The common scams are those that promise some service for a defined portion of the round being raised, not for equity or direct payment. I've seen accountants, lawyers, marketers, and brokers who will offer services based on a given number of points on the round. For instance, a broker will say "I'll intro you to my network, which is yuge!, and tremendous, believe me!, for 8% of the round." Then nowhere in the language of the contract does it say that the round has to come from their introduction. Lawyers will look over your docs or prepare them for a percentage. Accountants will manage the funds or help you redo your cap table etc for a percentage. Marketers will clean up your pitch, advertise, build you Exec summary and so on so they are as pretty as possible ... For a percentage.
I become very skeptical every time I see someone who wants a portion of the cash. This isn't to say that everyone who does this is a scammer. I know a few people including lawyers who do this legitimately. However their fees are only a little higher than reasonable and customary since they are taking on the work with the risk that you won't get funded.
The next set of scams all involve people who want to use your venture to enrich themselves non-monetarily. Things like taking credit for your success, marketing themselves or building their network based on your work. I've seen pitch advisors who just coach you on public speaking, go to every one of your pitches and pass out business cards. They don't really help you all that much but use you as a marketing vehicle. Sometimes these people can come clothed as investors, but it's rare.
There are others, to be sure, but none that I've seen where someone poses as an investor.