In addition to testing, you can estimate your levels based on past readings and what you have added. Fresh water added to replace evaporated water won't change the level of CYA. Fresh water added to replace water lost to backwashing, leaks, splashout etc. will lower the CYA level.
Adding Cyanuric acid, trichlor tabs and/or dichlor powder will raise your CYA. The pool calculator will tell you how much the CYA is increased based on how much you add and your pool size.