You can get the charset used by the current session via the session-specific global variables @@client_csname or @@client_csid.
You cannot programmatically access this information for other connections than your own, but in case you just want to collect information about all charset used by connecting client applications, you can use a login trigger to capture this information for each session that connects to ASE (and for example insert the details into a table which you can inspect at a later time).