^^ first of all drive the bike for atleast 10 minutes before buying. I am sure 60% of your decision will be made in these 10mins automatically. If its upto me, I will never buy a bike if it does not appeal me in first 10mins drive.
Rest i don't know how to check the bike by engine. There are senior members who can tell you about that. Better if you take any reliable mechanic with you while buying. And in case of used, only buy cd70. It will be better to buy a 2006/2007 model used CD70 in range of 30000 rather than buying 2011 model used chonda in the same amount...