…is check the logs for the unit, i.e. run "journalctl -u …" on the unit.
With v258 there's now a combined way to do this. If you add the -v switch to your "systemctl start" invocation, "verbose" mode will be invoked, which means the logs will displayed "live" covering the time span when the start operation is started until the start operation completed.
This has been a much requested feature – I figure some of you probably even looked for this feature personally already.