Controlling behavior can be emotional, physical or spoken. The controller attempts to mold you into what he wants for his personal gain and power. He might tell you how to dress, who to hang out with and where to go, and he might isolate you from friends and family. Controllers use humiliation or intimidation to get their way.
Flirting with other women is a red flag. It's disrespectful toward you, especially if he accuses you of being a prude or jealous for confronting him about it. Flirting with other women is a sign he may not have the ability or maturity to be faithful to just one woman.
Abuse of drugs or alcohol is a red flag. A person who is abusing drugs or alcohol isn't likely to give up his drug of choice for you. His addiction will be toxic to your emotional and physical health and to your relationship.
When the person you are dating frequently talks about his ex, it's a red flag that he has an attachment to the ex. The attachment may even be a negative attachment, but unresolved feelings are there. Take this as a red flag that he hasn't moved on and isn't ready to fully commit to you.
Everyone has a bad day, but a person who has frequent angry outbursts will cause you a great deal of stress. Uncontrolled anger can be a red flag of a potential abuser who may harm you emotionally or physically.