If the RAS client attempts a connection that fails, or if the RAS server doesn't start, the first place to check is the Event Viewer. By default, all server errors, connection attempts, disconnects, etc. are logged into the system log. Check there first--you might be able to determine the trouble simply by taking a close look at the log.

If a user is having problems being authenticated over PPP, a PPP.log file can be created to provide debugging information.

Hive: HKEY_LOCAL_MACHINE
Key: SYSTEM\CurrentControlSet\Services\RasMan\PPP
Name: Logging
Type: REG_DWORD
Value: 1 standard logging
Value: 2 verbose logging

Another handy tool is the Dial-Up Networking Monitor, which is launched at the dial-up client and shows the status of a session that's in progress. This tool will report information about the amount of data transmitted and received and the number of errors encountered in transfer. It can also show which lines are being used for multi-link transmission.