How do you know if the IRS got your tax returns by mail. I mailed them on March 7 2020, when I check the status it can't find it. Can I mail them again?

MAILED RETURNS

Per the IRS website IRS Operations During COVID-19: Mission-critical functions continue: The IRS is not currently able to process individual paper tax returns. If you already have filed via paper but it has not yet been processed, do not file a second tax return or write to the IRS to inquire about the status of your return or your economic impact payment. Paper returns will be processed  once processing centers are able to reopen. 

If you mail a tax return (or a payment) to the IRS, it is a good idea to use a mailing service that will track it like UPS or certified mail so you will know it was received.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. 

TurboTax will not know anything about your mailed return, and will continue to show “Ready to Mail” on your account.  TurboTax will not know that you put your tax return in an envelope and took it to a mailbox.  TurboTax does not get updates on mailed  (or e-filed) returns.

When the IRS finally opens its mail and begins to process mailed returns you can check the status on the IRS site. https://www.irs.gov/refunds

State returns have to be mailed to the state.

https://ttlc.intuit.com/questions/1899433-how-do-i-track-my-state-refund

The IRS is currently not processing mailed returns and hasn't been for a while. Do not send another copy because it will confuse the IRS and it will just end up in the pile of unopened tax returns at the IRS offices.

TurboTax gets no information about the status of your refund.
In normal times, the IRS says that most filers get their refunds within 21 days from the time their tax return is accepted.
Many returns are taking longer now since the IRS did not process returns for a while and there is now a big backlog of unprocessed returns.

When the IRS starts processing your return, you can track the status of your refund using this web site:

https://www.irs.gov/refunds

Here is an IRS Q&A with information about refunds:
https://www.irs.gov/refunds/tax-season-refund-frequently-asked-questions