Working at Gamestop, I've seen a whole bunch of sealing jobs. From new games to people trying to return them as "Unopened". Inidcators of a resealed game:
1. Shrinkwrap. Flimsy and thin. You can tell the difference right away. Most new games DO NOT use this type of packaging, and hving a game like this is usually a good indicator that somethign is wrong.
2. Fingerprints under the wrap. I know it could be somethign that was done at the factory, but in my experiences, if I see finger prints under the wrap and then open it, I find Toaster instructions and a Fifa 2002 disc.
3. Security seal. If it's not there, something is wrong. People get around this, however, especially with the PS2. You can open the case enough at the bottom to put the disc in there without even damaging the seal. NGC owners are out of luck on this one.
4. Dings or rips in the case. This is an obvious one that people overlook. If you get a new game, and it's beat up but was behind a glass case, either an emplyee screwed up or you're getting screwed. If the damage is around the spine of the case, then it's almost positive that your game was tampered with. In my experience, shipping problems always have damaged the front (indentations etc)
As someone stated, some games are shrinkwrapped, but there's a difference between facotry shrink and a amateur job. Look at a new GBA game. Flip it over, see theseal line on the back? How it's slightly perforated and looks kinda "folded over". Now look on the sides, there should be two more. This is because of a special machie that manufacturers use to seal the game by folding the shrink over the top and wrapping it around the back and then sealing it. Home machines use a long roll of shrink wrap that seals at the side and possibly bottom/top. There isn't a seal on the back. BIG MINDICATOR. Also, usually when wrapping something, the amateur will do it incorrectly and allow the seal to be puled to the side or even on the front cover of the game. If you see that, you know some foo' has resealed it. The best bet, honestly, is to go with your gut.