Ask from genuine curiosity, and make it a safe conversation so you will get an honest and useful response.
You may have thought that the response would be about the expensive gift or long walk on the beach. But it might be something much smaller or less ‘typically’ romantic.
It may be about the time you noticed that they were stressed and you brought them 2 coffees – just the way they like it. Or when you declined your parents’ Shabbos invitation because you knew your spouse just was too tired for a Shabbos there.
Be prepared to share some examples of when you felt loved. True examples, not those you think the other wants to hear.
You know your spouse best. Would they feel overwhelmed by the open-endedness of the question. Would it be easier for them to answer if you offered some possible answers, with the statement that they can of course add whatever they want.