How do you know if sciatica is herpes related for a fact or just a pinched nerve?

Get an eval: A simple history and Physical exam would be able to tell you. Unless you have an active infection of Zoster, affecting the sciatic nerve root/Dermatome it would be unlikely for Herpes virus to cause this.

See below: Sciatica refers to an irritated nerve in the spine or pelvis that causes pain down the leg and is typically not related to herpes.

Shingles?: Sciatica doesn't cause a rash. So if you have a rash you might have shingles. I would see your doctor to evaluate this.