Ok were to begin... 01 drz250(in australia) Out riding with the lads and had a bit of a rattle.. Then after an interesting corner or two had a fair bit of smoke come out of the exhaust... So did something stupid and kept riding for the day... So it looks like my cam tensioner has crapped out... Cam chain has jumped a couple of teeth and spewed oil into the cylinder... I spoke to the dealer and he assured me the cam chain tensioners don't fail.. But am just trying to work out after have valves re seated due to fouling wheat her I should replace both the tensioner and the chain or just the tensioner. Is there a way to measure the length of the chain?

I would certainly replace the chain at the same time. The chain is probably not that expensive.
It would be a good insurance move to to so.

Wouldn't trust the what dealer says all of the time,i got a 98 DR250R with the same motor as what's in the DRZ250's n they reckoned the parts are totally different!funny that coz my mate has the same bike as you and the cam chain,chain guides,tensioner,head n base gaskets are all the same just different part numbers(appart from comp ratio)!
im rebuilding mine n the chain costs $122.06 P/N 12760-13EOO,tensioner is $117.86 haven't got a P/N yet,mite wanna check ur chain guides too,their $55.46 an $53 P/N
12771-13EOO and 12811-13EOO
Hope this helps n good luck with it

yes the tensioner can fail, its the same as the drz400 unit that is known to fail
i put a manuel adjuster on my drz250, but i plan to replace it with a new oe tensioner