What do you use to verify TIN? Are there any API services that provide TIN validation?
I've searched up on Google and found a few, one of them being https://www.tincheck.com/, but I'm wondering if there are other alternatives.
I’m aware of only one authoritative source for this, the IRS’s own “Interactive and Bulk On-Line Taxpayer Identification Number (TIN) Matching Programs,” which are free of charge and described in Publication 2108A:
https://www.irs.gov/pub/irs-pdf/p2108a.pdf
As explained in section 14, “Enrolled users may TIN Match in one of two ways:
“a) Interactively – a user can submit up to 25 name/TIN combinations at a time during a session, and receive a response within 5 seconds.
“b) Bulk – users may download .txt files composed of up to 100,000 name/TIN combinations and receive a response from IRS within 24 hrs.”
There is more information in section 3.42.8.7 of the Internal Revenue Manual:
https://www.irs.gov/irm/part3/irm_03-042-008.html#d0e1571
The IRS allows third parties to act as authorized agents, and I expect this is what many companies offering TIN verification are doing. It’s up to you to decide if the value they add is worth their fees and the risks of sharing sensitive information (like SSNs for sole proprietors) with a third party.