USCIS will issue a receipt number to the petitioner (your employer) by snail mail and/or email.
The receipt number is a unique 13-character identifier that USCIS provides for each application or petition it receives. The agency uses it to identify and track its cases. The receipt number consists of three letters-for example, EAC, WAC, LIN, SRC, NBC, MSC or IOE-and 10 numbers. You can find it on notices of action USCIS has sent you.
Your employer should provide you with the receipt number . You can check status online at
https://egov.uscis.gov/casestatus/landing.do
For me it took about two weeks for my employer to provide receipt number. Things are likely slower during COVID19 situation, but USCIS is still operating.
You can create and online USCIS account and be notified of any changes to the petition by email.
When on TN status, my employer provide receipt number for the H1B application. And later the I-140 application.
The AOS, I-485 and I-765 filed with I-485 are technically your applications, not your employer. The lawfirm provide receipt numbers for myself and family of AOS applications.