For example, you might say something like, "That sounds like a fun plan for this weekend. Just tread lightly, and try to bring it up in conversation.
If none of these options are available to you, you could always ask your coworker himself or herself. 
Ask your friend to be discreet, and just say something like, "I was thinking about asking _ out on a date do you know if he/she is single?"
If you have a trusted friend in the workplace, you can ask him or her about the coworker you're interested in. You can also browse through some of your coworker's recent photos to see if there are any pictures of your coworker holding hands or cuddling with someone else, which might indicate a relationship.
Some social media platforms like Facebook have a designated profile line for relationship statuses. If you're friends with your coworker, you can check his or her social media for hints of a significant other. This can save you both a lot of time and embarrassment, and it can help keep your working relationship unaffected.
Before you ever approach your coworker about going on a date, you should ensure that he or she is actually single.