I am going to buy 150cc bike,what tips should i need to check if the sellers bike has a good engine or not?

Take it to an independent guy that is certified as a motorcycle tech. If you use a friend that knows bikes well, and he gets it wrong, it can put a strain on the relationship. The money you pay to have it checked out can save you a ton later. Obvious things to look for of course would be fluid leaking. The guy you're buying it from should run it for several minutes. This gives you a chance to see if anything is leaking. He could have totally wiped it down and detailed it otherwise just so you wouldn't notice. The sounds should be smooth, no heavy metal banging when the gears are engaged. Listen to how it runs at idle. When more throttle is applied does it still sound right? Is there a lot of noise coming out of the top of the valve covers? How does the chain look? Is it too tight, too loose? Is there heavy rust pitting on it that suggests it's sat for a long time creating places where the chain is weak, and no longer safe? How do the tires look? Any unusual wear patterns? Are they holding air? How does the suspension feel sitting on it? Can you touch the ground with both feet flat like you would when you stop? Do the control levers engage and disengage smoothly? Just some thingts to think about. Hope this help.

Listen to how it sounds make sure its not leaking and when you test ride it give it a lot of gas to make sure it gives torque and doesnt bog out, run it for many minutes so you can make sure it doesnt leak coolent or anything. ask a lot of questions for example. when was the last time you had the valves adjusted? (they require this once every 100 hours) how often did you change the oil? what work have you got done in the last year? how often do you ride it? people normally dont lie, but dont just buy a bike on impulse lol pay attention to the persons house and other bikes if the have them, because if the person looks wealthy they may have taken better care of the bike. once you get the bike you should imediatly change the oil and transmission fluid also check the coolent.

There is a seam, front to back, down the center of the crank case, wipe your finger along it for oil leaks, Listen to the start for grinding noises, there shouldn't be any.\ The exhaust pipe can be sooty but not oily. If the engine runs smoothly, I'd consider it. Look at the frame for cracked paint at the welded joints, Minor scrapes and scratches means it has been dropped or fallen, but if it runs good , consider them cahracter marks.

Listen to how it sounds make sure its not leaking and when you test ride it give it a lot of gas to make sure it gives torque and doesnt bog out, run it for many minutes so you can make sure it doesnt leak coolent or anything. ask a lot of questions for example. when was the last time you had the valves adjusted? (they require this once every 100 hours) how often did you change the oil? what work have you got done in the last year? how often do you ride it? people normally dont lie, but dont just buy a bike on impulse lol pay attention to the persons house and other bikes if the have them, because if the person looks wealthy they may have taken better care of the bike. once you get the bike you should imediatly change the oil and transmission fluid also check the coolent.

There is a seam, front to back, down the center of the crank case, wipe your finger along it for oil leaks, Listen to the start for grinding noises, there shouldn't be any.\ The exhaust pipe can be sooty but not oily. If the engine runs smoothly, I'd consider it. Look at the frame for cracked paint at the welded joints, Minor scrapes and scratches means it has been dropped or fallen, but if it runs good , consider them cahracter marks.