Look for the puppet service running on the agent (which can be accidentally started by simply typing "puppet agent" instead of "puppet agent -t"). The master's logs should tell you which agents started up, so as long as you have permission to log into the agents themselves, you can check puppet's status as a service via the following methods:
Linux: Type "ps aux | grep puppet" on the agent. This will tell you whether or not a puppet service is running. Alternatively, use puppet to disable its own service by entering "puppet resource service puppet ensure=stopped enable=false"; this will make puppet incapable of demonizing by accident again and will stop a daemonized puppet service if one is running.
Windows: go to the start menu and either search for "services" or go to Administrative Tools > Services (there are other methods; it depends on your version of Windows). In the services Window, scroll down to the puppet agent. The puppet service is automatically enabled on Windows machines and needs to be manually set to 'disabled' if you do not want it to start up automatically.