Whip the head off and have a look(its the only sensible way), the very simple check is to see if there is a nasty ridge at the top end of the cylinder, if there is then it needs reboring (make sure its not just carbon build up though).
whilsts the head is off it is welath worth checking the valves as they are quite prone to damage, bent valves and pitted seats.
Get this all right before fitting the engine and it will last years trouble free. Mine has done over 12,000 miles without fault. I have had to pop the engine out this week due to the exhaust stud shearing off whilst driving!!!
Baz