Question: how do you know if you've got low blood pressure?
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.