With machine polishing, some things to look for:
-sometimes polishing will open up air bubbles that the polishing compound gets into
-on handmades the pontils are usually removed
-surface is very spherical and "slippery"
-patterns can change (seams can open up) as noted above
-colors can change (ie: oxblood)
There are other things that can jump out at you, but those are the first few things for me. If it looks too good to be true, or if something seems a bit "off", have a closer look.