Examine thoroughly whether it has any scratches, damage and rust in any part of the motorcycle.
Check the year of making on Original Registration Card (RC).
Examine the engine and chassis number, and verify both are matching with the numbers on the RC book.
Before approaching the dealer, gain some knowledge on the model which you are going to buy. That may save you from the fraudulent dealers.
Check whether all lights, horn, choke, indicators and speedometer are functioning properly.
Take your friends along while going to buy. It is also better if you take a motorcycle mechanic with you as they are very much expertise of the pros and cons of a used one.
Always have a couple of bike enthusiasts and ask them to test drive. They may find the cons that you've missed to identify.
Finally test drive the motorcycle yourself and think that it will suit your needs in all aspects like seating posture, riding comfort, suspension and braking.