manually with refractometer when doing weekend water change
also last night as bought a new pincushion urchin and tested its water, my water, and it had to go into QT as LFS and I are .006 apart (us 1.026 vs LFS 1.020)
This is one of the ones that usually bites me. Normally pretty on top of it with water changes and keeping things dialed. Its when I get in and turn off ato gravity feed thats been getting me. This last time almost had real problems. Its about time for alarms and reminders around here.