Mechanical
Engine
Check engine blow best areas are dip-stick, blow pipe & oil cap.
Suspension
Extra noises, play in steering or wheels, axle noise on turning
Gear box
Gear shifting, clutch and pressure plate
Body
Paint
Original paint has a smooth shine
Accidents
Garders, front "patti", pillars, "kuppay", rear (crumble zones) are not damaged or repaired
Chasis and body for rust and general alignment.
Accessories for functionality and completeness
Documents for being original, complete, no theft cases, tokens paid, etc.
I can't explain terminologies or how it is done, it takes time and experience to learn.
Still lot of experts get cheated in market daily.
Please be very very careful these days