The question to be answered comes in three parts:
The first inquiry helps determine whether malpractice has been committed. Malpractice does not depend on "how nice" the professional was, or whether he returned calls, or was really communicative with you. What matters is what the professional did or failed to do. Is there an injury, loss or damage as a result of the act or omission? Depending on your response to these (and similar) basic inquiries, there may have been actionable malpractice.
You yourself are rarely in a position to know whether or not there was malpractice, although the best cases generally have mistakes that a lay person can recognize. The professional who performed the service will usually be unwilling to tell you that she or he is at fault. (They may not even know, recognize or admit to themselves that they are at fault. Attorneys are adept at finding reasons why others are at fault.) In fact, often your attorney has to hire an expert or consultant to help assess whether or not there was malpractice. Unless the facts are very clear, you may be asked to pay for the cost of that initial assessment by the outside expert who will eventually aid or testify for you.
If you retained the professional to work for you, or the professional actually performed work for you, the first question is answered. Often an issue arises over whether you might sue a profession who was involved in a transaction or law suit, but who was not working for you. Sometimes clients want to sue their opponent's attorney. This is not permissible under professional malpractice principles, but might be actionable under other proceedings.
If the professional deviated from good and accepted standards then there might be a successful suit. Deviation from standards is proven in Court by testimony of an expert in that field that the professional failed to do some act, or did an act which fell below the accepted standards in his field. Often such a failure is easy to understand, sometimes it is a fairly complicated failure, but at trial, an expert's testimony will almost always be necessary.
Usually the most difficult part of the case is proving that the deviation by the professional led to the damage. In attorney malpractice cases this requires proof that "but for" the deviation by the attorney there would have been a good, better or different outcome. This, in practice, means that the plaintiff will have to prove the merit of the case that the attorney botched. In the absence of such proof, plaintiff will not succeed on his case against the attorney. To recover for an attorney's malpractice, your malpractice lawyer will generally have to prove that you would have prevailed in the matter but for the attorney's malpractice. This sets up a two-fold inquiry-- first that malpractice occurred and second that you would have been successful in the matter had the malpractice not occurred.