Never! Well at least if you don’t have to. I’m not a fan of that question to be honest. I think it’s a bit naff and sometimes can sound insecure. I’ve never had to ask it because every time I’ve been in a relationship it’s been obvious and if it’s a serious relationship it should be rather clear what’s going on.
If you are unsure, early days or you think there is a possibility you have got the signals wrong and its just friends with benefits, there is first of all a timing issue. If you have been on just a few dates, don’t say a word! You just might have to suck it up and go with the flow and see where things take you (which is not such a bad approach to have anyway). A question like this too early could seem too enthusiastic or even a bit desperate.
If after a few weeks of seeing each other you are unsure and your gut instinct is failing you, try with cute little funny or sarcastic comments. They might be dropping the boyfriend word or making sarcastic comments where appropriate like “oh I will have to tell all those other guys I’m seeing that I’m off the market.” Make it fun and light and not a serious “where is this going” type of conversation. If you have to ask where it’s going, consider if it’s not going in a direction that you think it should.