To figure out if she a boyfriend before I start wasting my time or without just coming out and asking. I found it helpful to almost assume she does and add it into our convo..for example

Out at the bar: after some small talk..

"are you here with your friends or boyfriend or?"

or

If were talking about where she lives "Do you live with your boyfriend?"

Note: I ONLY do this when Im not completely sure, other wise Ill just keep the good vibe going, only If I suspect she might Ill use this..

And Most of the time she will say "I dont have a boyfriend"

This also works good because it shows you didnt come into the conversation out of neediness to hit on her, more in a friendly way. and can also be another topic of conversation. Then once shes says that she doesnt have one you can start to escalate..."oh no, is there something wrong with you? (in a playful way)."

Just a little incite to what has worked for me.