How do you find out if mom is getting the best possible care?

I think dropping in as often as possible is the key to keeping them safe and well. If you can put your dad in the same home, it might be a benefit for both (if your parents get along). That way you only have to worry about the incidental needs and stopping by. The stress doesn't go away but it diminishes and there is some peace of mind. Seriously consider the suggestion from MiaMadre. It might be time.

To answer your question, visiting her as often as possible lets the staff know that there is someone advocating for her. Since you are not able to go as much as you would like, is there a family member, friend, church member, etc. who could "drop by" unannounced? These people can also keep an eye on her and report back to you any irregularities. I know for a fact that patients who have a lot of "visitors" get the best care.
Bless you for being so concientious.
Lilli

From other posts you have created, it may be time to move Dad to a facility too. If possible it would be great to have them both in the same location.

With the other issues you are having with your father, it may be time to consider this so you will have the help that can "handle" him.

Be strong.

Is there a reason why your dad isn't also in the nursing home? I mean can't they room together and keep each other company?