Sorry for your loss. if he used Turbotax, you would need to know his username and password to accesss the account which would provide info on whether he filed or not. If you don't know his user info you can contact support. However, I do not know if they would provide it. Security concerns.
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you can call the IRS 800-829-1040 but you might need some 2022 or 2023 information so the iRS can verfiy your identity
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you can register with ID.me and then check to see if there is a return transcript
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you can file form 4506-T (free) to get a transcript of the return or 4506 to get a copy for a fee.
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the last two would return nothing or a blank piece of paper if the return was not filed.
. https://www.irs.gov/pub/irs-pdf/f4506.pdf 4506
https://www.irs.gov/pub/irs-pdf/f4506t.pdf 4506-t
