Use the "suggesting" feature on google drive. To do this, find the pencil icon in the upper right hand corner. Click on it, then select "suggesting". Now when you make changes, your partner will see what you suggested.
Use the comments feature. Highlight the text you want to make a comment about, find the "comment" button on the top right hand corner of google docs, click it, then type your comment. Click "comment" just below the text you typed.
Pay attention to citations! Does your partner regularly cite information in their paper? Do they have a reference page? To the best of your knowledge, is it properly formatted? What resources do you have to help you with this?