If the heads are off, the easy tell is the piston dish. The phase I engines have square cut outs on the pistons. The phase II engines have little valve cut outs. Basically, the phase II has less dish. Also, if you turn the crank, the phase I pistons will clear the deck height.
If the heads are on, it is a little harder to tell. Do you have any pics? Also, do you have the applied engine code off the timing belt cover, or on the rear side of it near the flywheel? It will be a ten digit alpha numeric starting with Ej25* *****