FIFA offers the following definitions when determining whether a challenge was careless, reckless, or with excessive force (which is the only question that needs to be answered to determine whether a foul deserves a misconduct card):
Careless means that the player has shown a lack of attention or consideration when making his challenge or that he acted without precaution.
Reckless means that the player has acted with complete disregard of the danger to, or consequences for, his opponent.
Using excessive force means that the player has far exceeded the necessary use of force and is in danger of injuring his opponent.
In order to determine these, one must employ the use of a few simple considerations to make the decision (these were instructed to me by a former FIFA referee, now the Director of Officials for the Canadian Soccer Association):
1) With which part of the body did the player make contact?
2) With which part of his opponent's body did the player make contact?
3) How fast/hard was the challenge -- how fast/hard did it need to be?
4) Did the player put his opponent in a dangerous position? (This would be reckless)
5) Or, did the player endanger the safety of his opponent? (This would be excessive force -- the danger is actual and real, as opposed to possible)
6) Did the player have a chance to play the ball?
7) Was there foul contact before the ball was played or after the ball was played?
Answering each of these quickly in your head should help you come up with the right choice in terms of misconduct (if applicable).