Pose it as an hypothetical, detached scenario, involving other people, "Like what do you think of..." "so and so did this.." [insert any romantic, relations stuff here] Gauge their opinion. If they respond negatively, joke it off. If they respond favorable, proceed to shift yourself, into the conversation. I mean, that's what I would do...but what do I know. If that makes any sense.
I feel like this is a teensy bit risky, as I know I might respond quite differently to a hypothetical scenario I know I'm not a part of than I would to a person telling me they are attracted to me.
My ex tried something similar before we started together. She mentioned how she was dating this guy, and though he seemed nice and all, there was something missing, and asked if she should still see him. As I had a crush on her (we had been friends for over a year at that point), I didn't want my feelings to cloud my judgement, and so told her basically to do what she wanted, but inside, I now felt certain my love would not be reciprocated as she was openly talking with me about dating other people and asking for advice on it.
It wasn't till long after we got together that she told me this was meant to show her interest in me (one of many failed gambits), and she was hoping I would say something about her dating the other guy and admit my feelings, but it had the opposite effect. It wasn't till a week or so later when she outright told me "I like you more than I should" that we started together (and even then, I was unclear and confusing in my messaging for the first few days).
I second
@Rodafina's suggestion. Honestly, that sentence seems close to perfect. Short, honest, to the point and little room for misunderstanding.