Most plays in every game are easily discernable as a hit or an error. There are usually going to be 2 or 3 that could go either way and it is a judgement call. The key is to use the same judgement for every player on both teams.
As stated, if it would have taken a great play to get the out, do not reward great effort by a fielder almost making a great play and call it an error. Also, if the runner would have been safe anyway, disregard the boot unless it gives them an extra base (as in bunts, slow rollers and balls in the hole)--sometimes this is obvious and sometime it is a judgement call. If throw is wild or first baseman drops the ball, if the runner has beaten the play, it is a hit and an error only if the overthrow allows an extra base.