There's a way to know if your friend list is still mutual.

[SuperZay]

But it requires a personal house.

Sometimes someone adds you to friend list but you never actually interact with them for months, and after while you start questioning what's the reason for them being there and if they still have you on their friend list. All you need to find out is to try to share a house with them!
(Social - Housing - Estate Settings - Private Estate - Shared Estate Settings - New Tenant - Click on their name - Yes - Close the window).

If you get a message like this



that means that they are still on your friend list (othewise you'd be unable to share because it's FL-only option), but they have removed you from theirs (I've asked and confirmed this).
In that case it's safe to remove them from friend list as well.

[Atoli]

...or you could just remove them without going through all this trouble if you haven't interacted with them for several months

An interesting find nonetheless
Thank you for sharing.