stock for stock a 230 has a flat top piston a 250 has a dish. Pull a plug and take a look inside the hole is another.But like deucecoupe said,Problem is the 230 and 250 shared the same block, so the only sure way is the distributor pad stamp.
Of course, the only real sure way is pull the head and measure the stroke!

Larry/Twisted6
[oooooo] smile
Adding CFM adds boost smile
shocked God doesn't like ugly.