how can i tell if the heads r cracked

well if it is leaking water from the head then atleast a blown gasket .to determine if it is cracked you need to remove head and inspet or have a machine shop check for cracks.

if the heads r cracked would it be good to get a new engine or just replace the heads

well if the heads are cracked by the time you repair everything that will need to be repaired. you would be better off to get a new engine