Observe his behavior and how he acts toward others. He should, as a general rule, be courteous and polite. Most guys will be nice to you if they are trying to date you, but if he talks meanly about his friends, is not reliable with them and has treated his past girlfriends inappropriately, that is likely how he will end up treating you. Avoid rationalizing his poor behavior with excuses such as "He just hasn't found the right girl yet." Remember the adages: "The best predictor of future behavior is past behavior," "A leopard cannot change his spots" and "Once a cheater always a cheater." These adages are based on observations of human behavior throughout the ages and have stood the test of time for a reason. Remember though, that you are looking for traits versus isolated instances of such behavior.
"Make a list and check it twice." Think really hard about what traits are important to you and write them down in a list. Rank the traits you've listed in terms of importance. Think about what would make you happy in the long run. If "good looks" is at the top of your list remember that as people age, looks can fade. Consider focusing on permanent qualities, such as honesty or generosity. Let's say your list is something like this: kind, generous, shared values, good provider and handsome. Rate these qualities in the order of importance to you and rate the guy you are looking to date on each of them. In addition to your own assessment, listen to what his friends or coworkers say about him.
Talk to him. If he seems to embody three or four of the qualities on your list, give him a chance. Go out on a first date with him. See what happens. Meet in a public place so that you are safe and there is little pressure to become physical after the date.
Ask him to talk about himself. You can get an incredible amount of information from someone just by asking. If you are flirty, you can ask just about anything without offending someone. You can say, for example, "Why is a nice guy like you still single?" His answer will speak volumes about how he sees himself and how he treats others. If you are looking for a generous mate, look at where he takes you for dinner, how easily he parts with his money and how he spends his free time. If you are looking for shared values, ask him what his views on a certain value important to you are and compare his answer with your thoughts on the matter. If you are looking for a good provider, ask him what his career goals are and assess if he is realistically working toward those goals.
Continue to evaluate him. If the first date went well, go out on a second date --- again in a public place. Carry on observing what he does and how he treats others. Listen to his opinions. If you like what you see and hear, date him more regularly.