How to tell if he attacked/bullied her or was he playing?

Our resident cat is a neutered semi-feral male who has lived with us indoors for a year, named Sultan. We just adopted another semi-feral who is female and spayed, her name is Peanut. We cannot pet either of them. We keep them separated when we're not home.

Peanut usually hides but last night she came out of her corner and trotted right past him in the living room because she thought he had gone to the other half of our apartment. He came right up inches from her face. She was cowering and hissing. Then Sultan cautiously tapped her front paw once. Peanut ran to go hide under the sofa and he chased her and attacked (?) her back with his front paws. I don't think I've heard him hiss or growl at her ever, it's usually her making those kinds of sounds because she's really scared of him. I don't believe his ears were flattened and I don't think his tail was swishing. Sometimes he even seems to talk to her in a special voice that we haven't heard before, it's not a meow, it's a gentle high-pitched talking voice, it's hard to explain. Anyway, he doesn't seem to be hostile or aggressive to her but the way her went inches from her face almost looked like he was trying to intimidate her.

How can I tell if he really attacked or bullied her, or if he was just trying to play with her?

He may have been meaning to play, or something more serious, but the fact she is afraid of him and ran could definitely have triggered an attack instinct in him. If she had turned and hissed at him instead, things might be different, but if she's too scared to confront him, you may have to figure out a long term set-up where they both have a good safe place to live, with all amenities and attention, but not necessarily interaction. Maybe someone else will have other ideas, but I wouldn't leave them alone together until such time as things are very different.