Often I'll introduce a character with the people they're closest too as well, so I might state their relationship status there too.
There are a few that I can't just sort as single or taken either ^^;
The two that first come up for me are Travis, the holpess romantic, and Adam. Adam tends to jump around a lot whenever he's present in a story.

If you're wondering about specific OCs feel free to ask. I know this one was pretty vauge