It's always a good time to test.
As far as to test for what
Here are the major parameters you should test for
PH, temp, salt level, No3, Po4, ALK, CAL and MAG.
Really any time, and long as you test around the same time for consistency. What you are looking for in testing, is if something is really out of balance. The test we use in the hobby are not exact and chasing a number will drive you insane. Use your testing as a gauge, take notes of what you have done, and eye the tank. When the tank look healthy you know you are doing things right.