A question about built in Oracle security

I have a web app that needs to work with a PowerBuilder app's database. This means sharing tables for user rights and other such things. The PB app uses Oracle's logons so the user id and password are passed in the connection string. To authenticate a user they are telling me to get the id and password and try to connect to Oracle and trap an Oracle exception if the connection fails because of invalid credentials.

Is there a better way to find out if the user is valid in the Oracle database? Our current database has our own table for  users so this is not an issue in the web app, but we are combining databases and would like to have the PB app maintain users, which means my tables wil go away. We don't want to maintain two ways of logging on.

Any suggestions?