I don’t think my mother (who has dementia) has filed her tax returns for at least four years. How do I find out?

Geobwright: Depending on her income level, she may not be required to file income tax returns.

If your Mom only has SS and a small pension, she may no longer need to file. There is a cap. My Mom and MIL both received a letter from IRS they no longer needed to file.

How to Find Out if a Parent With Dementia Owes Taxes

https://getcarefull.com/articles/how-to-find-out-if-a-parent-with-dementia-owes-taxes#:~:text=You%27ll%20want%20to%20request,made%20and%20any%20balances%20due.

Does she have income enough to have to file taxes?
You say that your Mom has dementia.
Who is her POA? Who cares for her? Where, or with whom does she live?

Basically the question here is, who is handling your mother's financial affairs if she has dementia?