I bought a used M1 2020 Mac Mini. I have a 1GbE switch. System info does not tell me if I have a 1 or 10GbE. I know that this is a custom addon when ordering so I doubt that this one has it but wanted to verify.
You should be able to get this information by opening System Information.app (found in the "Other" or "Utilities" folder) and clicking on "Ethernet". Does it not show under "Maximum Link Speed"?
FWIW, if I click on Ethernet in System Information, this is what I get. I am connected to a 1gb network however, and it shows 1000baseT in the ethernet section under Network. Caveat: this is from a 2018 Intel Mini with the 10gbe option.