Please help me with my mom.

My mother (58F) is an alcoholic, though she insists she is in recovery and does not drink anymore. I never see her drink in front of me or anyone else, and whenever we are out to dinner she orders a non-alcoholic beverage. However, I have real suspicions that she is still drinking or using some other substance. My reasons for thinking this are the following: in the evenings (or sometimes late afternoon, like today) her speech and mannerisms will change radically. Her speech slurs, her memory is greatly diminished, and her manual function seems to suffer (when she reaches for something it seems like she can’t find it for a moment before picking it up). This is something my siblings have also noticed. I have tried to discuss this with her in the past more than once, and each time she flatly denies using any substance (alcohol or otherwise) and has even suggested this could be something neurological, though she has never gotten that checked out. I don’t know if I’d rather be right, in which case she is still struggling with her addiction, or wrong, in which case something else is going on. I just want my mom to be my mom all day, not just first thing in the morning. I have no idea what to do next or where to turn for help.