I've used this a few times because I've found it useful at times when girls are flirty and then all of a sudden they mention a boyfriend. One time I actually met a girl, strong vibe, she was very flirty and called me hun, we get coffee two days later, and then she mentioned a boyfriend. I feel it would be socially uncalibrated to just out of nowhere ask if she has a boyfriend.

Here's how I most recently fit it into conversation fluently. I met this girl by opening direct and saying she was attractive. We text for a week or two and then she gets a boyfriend so we stopped talking. I ran into her a few months later so I started texting her again for about a week. I invited her to get jamba juice with me after class and I still wasn't sure if she was single. I wanted to ask her on a date at the end of it so here is how I fit it into conversation. I had mentioned a few days ago about a friend who cheated on his girlfriend and I was sort of stuck in the middle. I asked "Are you seeing anyone right now?" and she said no, I said "Well you can still say.... if he's drunk can he say it doesn't count as cheating?"

Another time I opened a girl at a concert by saying I liked her shirt and she was very receptive. She had a friend with her and I just randomly said in the middle of it "I need some relationship advice. Are you guys in relationships?" and they said no. I said "Well I guess you can still give your input. My friend is engaged and they've been dating for only 3 months. Is that too soon?" and they forget that I asked if they had a boyfriend because girls like to gossip like that.

Be careful because this could trigger bad emotions in her if she's had a similar situation and it turned out horrible. Try and jump off relationships thread as soon as possible. Don't say "What's your worst relationship ever?" or "Have you had a similar situation?"

Any input welcome

TL;DR Casually in the middle of a conversation, ask for their input on someone's relationship status (a friend of yours is engaged after 3 months) but before ask if they're in a relationship. If they say no then say "You can still give your input I suppose...." and if pulled off casually they don't even realize that you asked if they were single.