heres another imp thing u shud check besides what has been told before.
when the car is on petrol drive it up on a ramp. any slanted surface at an angle of 5 to 12 degrees, maybe more. press brake n when the car stops n is still in D gear. release the brake n do not press the accelerator. if the car climbs upwards or doesnt move at all the transmission is fine. if it slides back considerably while the car is in D then the condition of the transmission is not so good. remember to do this on petrol cuz on CNG theres a loss of power n cars do slide back (n shudnt be allowed to slide back cuz it ruins the transmission). all transmissions have a little play in them so if the car moves back a little, lets say an inch or half an inch, even then its okay. but shudnt slide backward more than that.