I 've decreased the cymbalta (duloxetine) from 60-30mg (1 week ago) to see if it's causing the parethesia i've had. how do i know for sure if it is the cause?

Is there a change?: Are you finding any change when you decrease the cymbalta (duloxetine)? Do you still have the paresthesia symptom, or is it tapering off? The main way to know if a medicine is causing a symptom, is to stop the medicine. But, i would not do this without discussing it with your prescribing doctor so s/he can help with this process.

More questions: I probably have more questions than answers. If you cut the dose and the tingles went away, then likely yes it was the cymbalta (duloxetine). If the tingles are still present the only way to know is to change therapy. Antidepressants are like house keys. Only one fits just right (controls symptoms / minimal adverse events). It's likely to be trial and error to find the right key.

Cymbalt: Discuss with your doctor if it is ok to take the medication at different times of the day to see if it makes a difference.