You will not be able to find out if someone filed a tax return using his Social Security number and name. The IRS will not tell you.
You will have to print and mail your tax return and include him as your dependent,
See this TurboTax support FAQ for how to print and mail a tax return when using the online editions - https://ttlc.intuit.com/turbotax-support/en-us/help-article/printers-printing/print-mail-return-turb...
The IRS will send you the tax refund that is on your federal tax return.
Within a year the IRS will start an investigation to find out who was eligible to claim the dependent. They will contact you and the other party by mail. The loser of the investigation must pay back any refund received based on the improper claiming of the dependent plus penalties and interest.
