Hindsight is 20/20. When going through a breakup, it's easy to find those early signs you shouldn't have been with him in the first place. So, save yourself the trouble and make sure you treat any instance of the following as a blatant red flag that you should reconsidering making this guy serious.
So you think he's the sweetest guy you've ever met? He's so polite with you – opening doors, texting you to see how your day went, bringing you flowers. You go out to dinner one night, the waitress mixes up his order, and he reams her out, calling her stupid at full volume, to her face. That lovely guy is actually an a**hole. Sooner or later that temper, lack of respect, and sense of entitlement will be taken out on you.
He lacks basic manners
I once was seeing this guy, Brett. Brett and I had gone out on a few occasions, and for whatever reason one night we ended up back at my place to watch some TV. I was still living with my parents, and Brett, who I wasn't so sure about to begin with, put his feet on my parents' coffee table. This was the first time he was ever in my home, and he put his feet on a table. That was the end of it for us. It's a small sign, but that's really all you have to build off of when you're just starting a relationship, and I took it to be a sign of things to come.
He makes fun of your friends
So he's met your friends, as a big “this is the guy I'm seeing” formal intro, or at some party you went to together. If he blatantly makes fun of any of your friends – calling them anything from ugly to stupid – dump him. This is either a very clear sign of a lack of respect for others, for you, or a sign that down the road he may attempt to isolate you from your friends or play a role in selecting them. He clearly has no respect for the people you've chosen to have in your life, so he doesn't get to be in it.
He lacks self motivation
Being with someone stagnant, or lacking motivation is a serious downer that either keeps you back, or breeds resentment if you're a motivated individual. If he's constantly talking about getting in shape but sits at home watching How I Met Your Mother and eating KFC, reconsider your involvement with him. If he says he wants to go back to school or go for that promotion at work and does absolutely nothing to achieve these goals, and deadlines pass him by, see above. Being with someone who is undisciplined, unmotivated, and lazy will probably cause you to become the same. Instead, find the guy who gets what he wants (you!), does what he sets his mind to, and isn't intimidated by anything.