The first measuring stick is the level of attention. If you feel that, the person is giving you an extra bit of attention than the other then feel that he is interested beyond the point of just flirt or just friendship. The feeling of touch is also important, if you feel that his touch is prolong and not your just like friends then feel that he is interested in serious dating with you.
A flirt often expresses shallow feelings, which are mainly fun and lighthearted. Whereas if a man is serious then his feelings and expression will be far more, genuine and deep rooted. As physically, if he touches your arm and holds those tight for a long time, then believe that he is not a flirt at all, rather he wants to go beyond the casual boundaries with you.
Eye language is another most important measuring stick in the argument. In case of casual friendship and flirting, the eye contact is usually for shorter period, mainly during face-to-face conversation, whereas in case of Dating service, if the man maintains the eye contact for a longer period with an intense emotion in the eye, then you must believe that he is not a flirt at all.
Intimacy in conversations is the next step to look at. Flirting involves a range of activities from conversations to gestures and touch. The actions can be either playful or more intense in their expression. If the conversation goes a step forward than the usual friendly boundary then think that there is more to it. Like, for example, if the person shares personal events and asks you questions on emotional issues then he can never be a flirt, rather he is looking for a more serious relationship with you. However, there are no such solid definitions of flirting vs. dating service because it is very subjective in nature; it is up to you to understand the difference between true feelings and flirting.