Well,among major inspection (because of which baleno is notorious)is its engine.Before you buy it you will have to see its exhaust first,is there any remanent of oil or extra soot,stay away.
Start the engine,listen to the sound of engine if any doubtful sound like karr karr on higher rpms,stay away.
Run the car on low speed with low rpms in 2nd gear on CNG,give it a sudden accelerator press and listen carefully is there any sort of PHUKK sound or a misfire,stay away.