How do you know if they are attracted??

I believe it has something to do with the whims and/or recommended things to say when you press on a sim that you are not controlling. If you are not familiar with this, I am sure you will be once I explain. It is when you are controlling a sim (just like regular) and when you click on another sim to interact with them. Before seeing the sections of things to say (like Mischeif or Friendly), there would be a couple of things to say that, I believe, the controlled sim would like to say (or do) to the selected sim. Like if your sim has met another sim and interacted with them for a bit, something would pop up in either the recommended list or the whims section informing to you that the sim you are controlling wants to flirt (or do something else romantic). Usually, in the sims, the game would not recommend your sim flirting with a sim the same gender (for some odd reason; I fully support the LGBTQ+ community). You would have to start that part on your own, but once the game learns that you are romantically interested in that sim, they would recommend more romantic interactions.

Now, onto whims. Whims are also things that your sims wants to do, usually based on the trait that your sim has. Say, if your sim is romantic, they would most likely be found wanting to flirt faster than other sims. If your sim is noncommittal, they would take a longer time to want to flirt with someone. If your sim is both, well, I guess your sim is successful at love, but doesn’t show it very often. So, if a whim pops up that has something to do with loving another sim that you are not related to in any way, there may be a big chance that your sim has is romantically interested in someone.