Unless you mail certified with return receipt or other tracking service (which is recommended for anything mailed to the IRS since that it the only proof of mailing that the IRS accepts) then there really is no way to know if the IRS has even received it.
Since mailed returns must be entered into the IRS processing computer by hand there might be long delays for any mailed return.
**NOTE*** Due to the Corona virus the IRS is operating short staffed. All field offices are closed and some processing centers are all but closed. Any tax return that requires hand processing will probably have long delays.