However, a gambler might say he will gamble for fun and still end up not gambling for fun when he starts gambling. One can know a responsible gambler if he starts gambling.
That's true. I've felt that way a few times whenever I go through a bad gambling day. Gamblers can quickly change how they feel once they're done, and it's always easier to emphasize the bad parts of your sessions.
For the main question, i've done both of them several times, and mostly gambling for fun up to this point. On a positive note, it's helpful to experience both the good and bad because you can use that mistake to remind yourself whenever greed takes over.