You will need to be able to access the central admin web site. If you don't know the URL and port and you can log on to the server, you will find an icon for central administration under all programs.
If you are able to access the site go to System Settings > Manage servers in the farm
in the Servers in Farm page you should be able to determine where the SQL Server is from
Configuration database server. If the instance name is
[name of server]\SQLExpress it is very probable that this is a single server installation. But we still cannot be totally sure. If it is anything else, even just the name of the current server, then you are safe and it's a Farm installation.
Just to be clear, a Farm installation can all be done on one server and the databases can be moved later and additional servers can also be added later. But a "single server" installation automatically uses SQLExress. The term "single server" is misleading.
If it is not documented some where, you will need to reset the Farm Passphrase. You do that by using the SharePoint PowerShell console with these commands:
$passphrase = ConvertTo-SecureString 'Your passphase text goes here' -asPlainText –Force
Set-SPPassPhrase -PassPhrase $passphrase -Confirm
Make sure you document it so that it can be found later. You will need this to join another server. Try joining the new server to the farm using the farm passphrase.