the only way to tell is to pull it out and look at it.
the impellers on the old plastic ones are tan or orange in color.
for a short time bmw upgraded to stainless steel (good).
then they upgraded again to a higher quality synthetic composite (good). these are the ones that are currently available.
there are aftermarket ones which are metal, but are cast iron (bad). i have a brand-new in-the-box never used metal cast iron one (Graf) which i will sell on e-bay because i have decided to go with the newer composite impeller.
read this for more info: http://www.bimmerforums.com/forum/showthre...p?postid=931023
the cheapest place i have found for a water pump is http://www.eap4parts.com
($67.95 for new OE composite, $41.95 for the Graf cast iron)