Check on your gateway web page under Status->Gateway Scripts–>Tag Change:
It will show last time each script ran and whether it succeeded or failed. Failure messages will show in log below.
You can also add your own logs there by adding code like this to your script:
# Get logger with name 'GatewayEventScripts.TagChange' (you can name it nearly anything you want).
logger = system.util.getLogger('GatewayEventScripts.TagChange')
# Log your message. You can include variables in log too.
logger.info('your log message with calc value: %i' %(calcValue))