I know this may seem like a "dumb" question

but how can I tell the difference between my yorkie playing or attacking my bulldog??? He'll jump at her face and nip at her if she has a toy he wants and they'll later play "keep away" with the toy. But I'm worried he may be attacking her and if I don't correct it he will think it's ok and this behavior will progress...he does bark and growl while jumping at her. My bulldog doesn't "retaliate" she'll either just lay there and take it, play keep away, or nudge him with her head. Any suggestions are appreciated!