how can I tell who a photo has been sent to

It depends how you sent it.

Email? Look in your sent items list.

Messenger app - look in all your chats.

How did you sent it?

Sent? You cannot.

Shared? Yes. Open the shared album and click on the person icon in the circle.

If you have any shared albums, they will appear in the left menu in Photos on the Mac. Or, by tapping albums on an I device and looking for shared albums.