terminal – Homebrew running service shows error, how to debug this?

When I run brew services list one of my services state shows err.
brew services restart elasticsearch@6 for example stops and restarts with no error output and shows:

brew services restart elasticsearch@6
Stopping `elasticsearch@6`... (might take a while)
==> Successfully stopped `elasticsearch@6` (label: homebrew.mxcl.elasticsearch@6)
==> Successfully started `elasticsearch@6` (label: homebrew.mxcl.elasticsearch@6)

But list still shows err. There’s something wrong but Homebrew doesn’t tell us what it is. How to fix this problem?