I know this is about agents, but ultimately you'll be dealing with the publisher. So if you connect on an agent, or a publisher, check in their guidelines or FAQs page and see if they're heavily into the author marketing themselves. Or the publishers asking about your platform, social media skills (Facebook, Twitter, blogs. etc) and want to know about all your connections to the public, I would be leary of, because those are the type of publishers that are relying on your fan base as an extra boost in sales.
Any publisher you deal with, if they have good distribution and marketing and all that stuff that should come with the package, they'll be confident in selling your work, regardless if you have a fan base/platform already or not.
Also be careful of agents trying to send your work to places you can submit to yourself. You want an agent that is going to get you into a place you can't get into yourself. Another warning sign the agent doesn't have all the right connections on their end.
Make sure to watch for those too. To me, it holds up a pink flag (maybe not red so much) but some gut feeling that, maybe this one isn't so good.
Now some agents, like agent Elizabeth Kracht from Kimberley Cameron & Assoc. (now this is just an example of what I'm saying) There's been a few of her sales from Diversion Books. Diversion Books has an open door policy of unagented writers, which means you can submit directly.
I'm not saying she's a bad agent. But if you don't see the agent making "big" sales either, then what's the point of giving 15% to an agent (have your money filter to the agency first, then you) knowing in your heart you could have maybe done it yourself?
So someone with that mentality can come along and say, agents aren't necessarily the best way to go either. It all depends. To me, if you don't have a really good agent vs. an okay agent or worse, a bad agent (cause bad agents are worse than no agents, believe me) then what's the point?
You need to taylor the road you want to take. There's a billion other things in the agent world. Like for instance...what if you start to have bad communication with the agent and this agent is very reputable? So you'll want to have good communication too. Not just say, they're the best. They might be the best, but not the best for you.