Clinical pic & labs. Your endocrinologist should be able to give you a fair estimate of what will likely happen based your clinical picture (ie. Are you post-parting, getting over a cold, famiy history, tpo antibodies ect.). Then intermittent monitoring of blood work will give a pattern that can confirm the direction you're heading.
Blood tests. The best way to know about the status of your thyroid is to do regular blood tests, if you are over or under medicated the blood test will reflect the adjustments that need to be made to keep you in shape.