depends, sometimes it depends on the quality of the paint, but even the autentic ones have bad paintjob.
some pets, as the recent ones have behind their heads, or legs printed LPS
which proves they are original, but the old ones don't have those.
if you mean how to recognize them only by looking at the picture, as when searching in ebay, it's harder, but sometimes is good to check the seller other products, if some of the other pets he sells look rather weird or fake, then you must be careful, also, some pets that are fakies have mixed bodies and heads, sometimes they even have mismatched parts!