How do you know you aren't already just a frozen brain in a liquid N2 canister, whose scan was uploaded to a simulation server with severely restricted outside

How do you know you aren't already just a frozen brain in a liquid N2 canister, whose scan was uploaded to a simulation server with severely restricted outside communication? Just a word of caution, however: if things around you seem to be reverting to an earlier era, get some Ubik, quick.