Look at how he treats relatives. If you see your partner talking to his parents in a disrespectful manner or verbally criticizing everything his or her siblings do, then this could be a red flag for you. If you see that your partner often neglects time with family or harbors bitterness towards them, then you should ask yourself if you want to marry this individual.
Consider how your partner manages money. If you're a person who believes in saving for the future but your partner spends most of her check at the casino or on unneccessary things, then you need to take this into consideration.
Take a look at how he talks to you. If the language is verbally abusive or damages your self-worth on a constant basis, then more than likely this behavior will continue after you're married. This is a sign of trouble because the verbal abuse can turn physical over time.
Examine your partner's commitment level. If you've been dating your partner for a few years but she's made no real steps taken towards the goal of marriage, then maybe she doesn't want to make a commitment. If you've noticed patterns of infidelity and your partner shows no respect for the relationship, then it may be time to move on.
Think about your partner's attitudes. Is your partner always looking out for himself rather than the relationship as a whole? Does your partner blame you for many problems instead of taking responsibility for mistakes and questionable actions and apologizing? If this is the case, then you shouldn't consider marriage to this person.