You never need to ask a female if she is single or not. If you're interested in her, approach her and strike up a conversation. If you like her and want to ask her out on a date, go ahead and do so. She'll say yes if she wants to date you. If she doesn't, she'll tell you so. The only time this might not work is if you're both busy with other dates already. If that's the case, just forget about asking her out directly. Send her messages of your own instead. Show interest in what she likes and how she lives her life. This will make her feel important and give her hope that one day you might actually ask her out. So, going forward, always assume that a woman is married until she tells you otherwise. Don't ask directly, but rather find out by reading between the lines.