Watch for nested loops. That's pretty much it. Well that and be aware that if you have two nested loops then one of them had better be looping through log(n) the elements of the other or less.

The most common quadratic algorithms have two loops that both loop 'n' times, or the inner loop runs between 0 and n times. Both are quadratic - the second case is twice as fast, but still quadratic. In some cases the inner loop is not on 'n' at all but is on something that is typically proportional to 'n'. That's just as bad. That is, O(n*n/100) is still O(n^2).