Where possible, always take a friend that has experience on dirt bikes when you are going to view. They will be able to point out potentially dangerous or expensive faults and recommend whether or not to make the purchase.
Any person looking to sell will not mind you checking over the bike on your own if they are completely genuine. If they start playing up over it then I would walk away immediately.
Having a friend or even a professional biker on hand may save you making a huge mistake and maybe your life too. Never rush into purchasing a dirt bike, the right one may just be round the corner.