Removing dark wood stain from cabinets

what is the best way to dark wood stain off of my cabinets not sure how long it has been there...

Can't guarantee but you might try: http://www.wikihow.com/Remove-a-Stain-Using-Common-Household-Ingredients

Depends on what type wood it is and what stained it. If it was stained before varnishing the only chance is to sand it out. Usually this is easy enough if its a tight grain wood like oak but the stain may have soaked in farther on pine for example. (although pine is easier to sand)

Anyway, get some rough sand paper (like 150grit) sand with a sanding block until the stain is gone then sand the area with 250grit paper to smooth it off.

Good luck.