If you are actively working on your Function then all invocation and logs will stream to your browser in near-realtime. Here's an example:
✗ curl https://my-domain-1234.twil.io/log-demo?arg=value OK%
As you can see, Twilio Functions streamed the start, end and console.log output of my function into the Twilio console.
If your Function returns an error in response to a Twilio Webhook then you will find this under the Debugger. Note: The debugger also supports searching by URL, so you could search for all errors/warnings relating to your function.