Contests for essays, pictures, recipes, etc. are the ones that are judged, and you would have to read the rule regarding how they will be judged. They all differ.
I rarely enter sweepstakes by mail. I do most of my sweeping online. Entries that are mailed in usually each get a number put on the envelope (they are not opened). The number then gets inputted into the computer and added into the online entries. When the sweepstakes is over, the computer randomizes all the entries for the winner(s). If it happens to be a mailed in entry, they pull the envelope and open it to see who to notify as the winner.