No matter what, you need to report the abuse you have witnessed to management. This will bring the abuse to their attention. It will also let them know that you are paying attention to what is going on.
Nursing home facilities are very leery when it comes to lawsuits, so if they know that you have seen or suspect abuse, they are going to be much more vigilant in monitoring the situation. If you have a loved one who was the victim of the abuse then you might also consider removing them from the home as quickly as possible