The bore can look fine (i.e. undamaged) but still be excessively worn or, more likely, gone oval. A check at top, middle and bottom of the cylinder, and parallel to, and at 90 deg from, the crank with a micrometer is the only way to be sure. If that checks out OK, put the rings in the bore and measure the clearance of the gap bewteen the ends of the rings. The service limits will be in the manual. Finally, as oldbmw says, fit the rings to the pistons and check the clearance between ring and groove with a feeler gauge.
To be honest, if you have stripped it down this far, new rings would be the minimum I would go for, even if everything else checks out OK.