Last tested: Sep 12, 2018
All Looker-hosted customers' logs will be in UTC. This is not determined by the location of the server, the Looker application timezone, etc., but by the timezone that the server is running on. For hosted instances, this will always be UTC, which will look like this:
2018-07-24 01:06:20.626 +0000 [INFO|cc3f2|source]
If you host your own instance, the timezone of your logs can be found in the last 5 digits of the log entry timestamp, for example:
2018-07-24 01:06:20.626 -0700 [INFO|cc3f2|source]
would be 7 hours behind UTC, which is PST.