Heroku: How can you check Heroku Error Logs?

Simply use heroku logs to display the last 100 lines of your logs.

Or to tail the logs in real-time:
heroku logs -t

Heroku only saves the last 1500 lines of logs, which you can access using heroku logs -n 1500. If you want more logging, check out Heroku add-ons like Logentries or Papertrail.