If it looks OK without any gouges or scratches then it generally is OK, but the only way you can properly check is by measurement, either with a micrometer, or the traditional way by using a new piston ring and measuring the closing gap with a feeler gauge...you'd need to know what the tolerances should be, however, which you will need a service manual for........but I would say if it looks OK then it is OK and fit the new piston.
You should also hone the bore if you're putting in a new piston and rings-you can do this at home using a glazebuster in a drill, but failing that you can use a very fine grade wet'n'dry paper with oil rather than water.
This is the proper tool:
http://www.justoffbase.co.uk/Spare-Set-Of-Coar se-Stones-120-56250-Draper-YCH51-177;jsessioni d=ac112b791f4303e944ed39264c8daeb7abd6b08890f3 .e3eTaxeKbh0Te34Pa38Ta38LaN10?sc=9&category=15 8 which you can get in most good spares places-it's purpose is to roughen up the bore so the new rings can bed in properly.
You can get away without honing, but if you've got this far it'd be nice to finish the job properly!!