How do you know if you have hypertension?

Blood pressure. You have your blood pressure recorded several times over a few days and if it is predominantly elevated from the normal range, you have hypertension.
Check blood pressure. Hypertension is a silent disease. There is usually no sign or symptom of htn. The best way to know if you have hypertension is to start keeping a blood pressure log book. BP > 140/90 mmhg is considered hypertension. BP between 120-139/80-89 mmhg is considered pre-hypertension.