How do I find how much has been paid to my 2018 tax liabilities in estimated tax payments? My mother made the payments on my behalf but is unable to find the documents.

Unfortunately, we don't have access to that information.

You need to see if you can determine the amounts through your mother's records. Payment to the IRS should be shown on her bank statements or credit card bills. 

Otherwise, you will need to contact the IRS to see how much as been paid.

You can request a transcript, Get a Transcript or call the IRS 1-800-829-1040.

Please be prepared. The response times from IRS may be lengthy. This is a very busy time for them and they started tax season behind because of the shut down.