To me, the children interactions on social media could have been a good indicator. But now that Piper left Instagram, it’ll be hard to tell. We actually only see PM for awards ceremonies, so i guess we will have to wait for the next awards season.
Also, if she’s indeed in The Crown, there will be filming, sightings, interviews, parties, so sadly we’ll have to see more of them together but the status of their relationship will be easier to decipher.