Question: how to know when your blood pressure is low?
Answer: Dizziness or lightheadedness.
Nausea.
Fainting (syncope)
Dehydration and unusual thirst.
Dehydration can sometimes cause blood pressure to drop. However, dehydration does not always cause low blood pressure. ... 
Lack of concentration.
Blurred vision.
Cold, clammy, pale skin.