I have found that one of my main problems with a female is having a really good conversation and asking her to hang out sometime then finding out that she has a boyfriend.
Sometimes I will use a direct "You're cute, let's talk" approach which is usually easy to ask if they're single.
Lately I've been doing the Mystery Method indirect approach which would be awkward to just ask if they're seeing somebody if you're trying to show disinterest. I try to find out as soon as possible if they have a boyfriend because we all know that girls will flirt and show IOIs regardless.

So here is my "boyfriend test" routine that I've field tested:
I ask for an opinion on a situation a friend of mine is in where his girlfriend doesn't trust him to go to a concert because he has cheated in the past. I think they shouldn't stay together if she can't trust him. I then casually ask "Are you in a relationship?" "Yes" "Well you know how important trust is in a relationship"
I've used different stories, such as (true story) this girl I know has been dating a guy for 3 months, they're already engaged. Is that too soon? Ask them if they're in a relationship and if they say yes then ask them for how long or something.

If they say they are single it leaves open the opportunity for a neg like "I can see why. You seem like a lot to handle"

I sometimes open with this routine but most of the time I'll start with another routine and feel a good connection and have to find out if she's single. I somehow find a way to fit this into a conversation.
Do you like music? - Ever go to concerts? - I went to one last weekend. My friend couldn't go because his gf wouldn't let him.
I went to see that new movie this weekend with a group of friends. One of them is crazy about this guy who lives 700 miles away. - Long distance relationship opinion?

Good routine. Thanks for sharing.

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Before she can respect you, you need to respect yourself.

Even simpler thing to do is just throw in a line like, your boyfriend is totally gonna kill me isn't he?
If she says yes and it's true, then stay or go, whatever you want.
If she says yes and she's lying, if you can see the cues then call her out on it if a cocky funny way:D
If she says no, bada bing!

Yeah I figured this out by practicing my indirect as well. Instead I neg regardless.

First i'll get their female opinion on something, most likely dating related. Then i'll ask my target anywhere within the opener "Wait, are you dating?" Whether she says yes or no, my response is the same. I'll butt her out of the convo, saying "her opinion doesn't count", while turning my back on her and focusing on the obstacle or just showing willingness to walk away.

Just because you're going indirect doesn't mean you can't get alpha with her~

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Two words: Carpe Diem

One method i got taught for screening for a boyfriend went as follows

I was talking about my passion topic (cooking) to a 2 set and said "Can you girls cook". They replied with blah blah blah and i said "apparently cooking schools are a great place to meet hot guys".
The shorter one turned to her friend and said "Thats where we need to send you to find a man"

BUSTED!

Her friend dobbed her in as single, i knew it was the green light to move in for the kill then