Everyone knows that overambitious marketing statements like "I guarantee to make you rich if you invest in my fund" are a huge red flag. But beware of underambitious statements too:
These days, especially among professional value investors who appear in the media, I hear a lot of "this is a great time to buy" and "the market is very undervalued" and "Finally I can own all these great companies I've always coveted but were too expensive." The subtext of these public declarations, of course, is that the investor is looking to keep his existing clients and add new ones.
All these statements may be true, and there is nothing evil about a professional investor marketing himself, but remember the goal of active investment management is to earn, after fees, a higher risk-adjusted return than the client could earn on his own. If the client is completely passive, he could simply invest in an index fund. So the statement "the market is undervalued, it's a great time to buy" does not in itself justify active investing because the client is perfectly capable of acting on that statement without paying a fee.
If the client is a little more sophisticated--enough to separate a great company from a mediocre one--then he can simply pick a basket of great company stocks on his own. In this case, the statement "finally I can own the great companies I've always wanted to own but were not undervalued" does not in itself justify active investing because again, the client can act on it without paying a fee.
In order to justify his existence, the active investor must be much more ambitious. He cannot simply identify that the market is undervalued, he cannot even identify that the world's best companies are undervalued. His goal MUST be to identify those opportunities that are the MOST undervalued, on a risk-adjusted basis and with the required margin of safety. He has to do something the client cannot, and he must do it well enough to justify his fee.