It deprnds on the situation. To start conversation, I would use: (name)さんはコンピューターに興味がありますか。 After talking about computer, for example, she/he knows very well about it or she/he seem to be interestet in it, to make it clear or when I'm a bit surprised, I would ask: (name)さんはコンピューターに興味があるんですか。 I hope this helps you.