dipstick

from WordNet (r) 3.0 (2006)

dipstick
    n 1: a graduated rod dipped into a container to indicate the
         fluid level

from The Collaborative International Dictionary of English v.0.48

dipstick \dipstick\ n.
   a graduated rod dipped into a container to indicate the fluid
   level; as, to check the oil level in a car with a dipstick.
   [WordNet 1.5]