What are the signs of bad piston?

Answer 1

Blue smoke out of the tail pipe, low compression, excessive blow-by, excessive motor oil consumption, loss of power.

A more detailed answer

If all your piston rings are badly worn you'll have blue/black smoke coming from the exhaust every time you accelerate and your engine oil dipstick will show the oil being used up much faster than usual, like you will have to top-up with a pint of engine oil every 300-500 miles. If only one or two pistons have worn-out piston rings the symptoms would not be so bad.

It all depends on how well the engine has been driven and maintained. On a modern engine, say less than 15 years old, if the oil was changed regularly and the engine was never thrashed (over-revved) then it is unlikely you'll get worn piston rings until the engine has done 150 thousand miles, or much more if it's a diesel.

Note

On modern engines with overhead camshafts, if the valve-seals are worn you'd also get blue/black smoke coming from the exhaust every time you accelerate and would be burning up engine oil fast. Even if the engine was driven well, and its oil was changed regularly according to the recommended service intervals, its valve-seals are still liable to wear out faster than its piston rings. How long the valve-seals last will depend most of all on the quality of the materials used by the engine manufacturer and, to a lesser extent, on the engine size: on some makes of engine the valve seals could normally need replacing every 100 thousand miles, on others they could last 300 thousand miles or more.