Just go on Xbox.com and check to see when the achievements are unlocked and if they don't add up then it's probably modded, or if the games have 1,000/1,000 than that's also a sign that it is modded.
To check if a tenure is legit it's pretty easy just go on Xbox.com and see when the games were played, it'll usually say something like "last played 6-9-2008". Also another way to check the tenure go to Halowaypoint.com and check the accounts Halo 3 stats, if it has any it will show you when it was first active on that game, for an example "First active 9/24/2007".