I received a letter stating I did not pay my 2019 taxes but I did send a check. is there a way to check that?

You have to send them a copy of your check and maybe bank statement showing the payment.  Maybe they applied it to the wrong year?  Did you send it with the 1040V voucher or was it a 1040ES estimated slip?  Seems very late to be getting a notice about a 2019 tax return.  Maybe they changed your return and you owe more?  

Check your IRS account.  Request a transcript from the IRS
https://www.irs.gov/individuals/get-transcript

You will have to look at your bank statements to see if the check was cashed by the Payee.  If you have a copy of the statement showing the payment then contact the IRS using the phone number in the notice you received.  Your cancelled check and statement is your proof of payment.