Most players have a range of hands they will play, usually much expanded in late position or if short-stacked. You need to work out what sort of player they are. In low stake and freeroll tourneys, especially ones with a big field you get at lot of players that will punt all in with nearly any two cards near the beginning. But just because you observe this behaviour early on doesn't necessarily mean that the player will continue to be all that loose, however. Remember that if you have six or seven all-in bets in front of you, your pot odds are pretty good for any hand.
What is most useful is finding out how they play specific hands. For instance, I've known some players who would make a standard raise of 3BB with a large variety of hands, 4BB with pocket kings and 5BB with aces. Look out for this sort of player: knowing what your opponent is holding is the biggest edge you can have in poker.
Others are much harder to read. This is why mixed play is superior to just playing tight. Using information bets on the flop is often a good play. Say you hold As Jd in middle position and the flop for a 3 way pot is Ad 6s 9s. A raise of about 1/3 of the pot will, as well as giving you a bit of fold equity, provoke many players into giving away information. There are quite a number of drawing hands that might be out there, so someone with trips will likely make a substantial reraise. Two pair is a likely a small reraise, especially if he holds A9 or A6. AK or AQ would probably flat call or min raise, unless they're a maniac: but this sort of player usually raises all in preflop with AK or AQ. If you get reraised you're probably beat but have a cheap fold. If you get flat called you are also probably behind and need to improve on the turn to continue.
There are lots of ways to get information; but remember that sadly, most of them are little more reliable than a crystal ball.