Your question is a bit unclear because I cannot determine whether you are positing a situation in which it is known that there has been an abusive experience or if you are inquiring about how to elicit the fact to begin with. However, we can examine both possibilities, each of which is, of course, quite dependent on situation and client, and which is also based upon what therapeutic underpinnings you are inclined to rely upon.
In the first instance, in which abuse has already been revealed, the point is generally to elicit further information in the hopes of placing the client in a position in which he or she can reveal all and be able to move on or to place the client in a situation in which he or she can "reframe" the abuse in a way in which to feel more powerful and less victimized. Often this is painful for a client, so my inclination would be to keep questions very general before homing in on specific details. Another way to question is to place the focus on the client's feelings, rather than on the specific details. For example, if one's intention is to help the client reframe the abuse, one might ask,"How did that make you feel? and then follow up with questions such as "Now that you are an adult with power, what would you do or say to this person?" This allows the client to understand that he or she can no longer be victimized.
In the case in which you suspect childhood abuse, but none has been revealed thus far, there are clients who are anxious to "get it out" and clients who are quite well-defended. So it is very situational, and a good therapist must have some sense of the client's feelings and attitudes. One approach is to "bury" a question on abuse in a series of questions about childhood, so that the client is primed to answer before encountering that particular question. Another way is to ask very general questions, again. For example, one might ask about happy memories from childhood and unhappy memories from childhood. This might lead naturally to a discussion of abuse. Also a possibility is to tell a client that he or she is exhibiting behaviors that are consistent with a history of child abuse and to simply speculate why that might be so, leaving the door open for the client to discuss this.
Whether or not people actually remember childhood abuse, have buried it deeply, or are susceptible to "false memories," a good therapist must be careful not to implant any memories of events that never occurred. This leads to numerous problems for the client and the therapist.
No matter what school of thought you subscribe to as a therapist, it is always best to proceed cautiously in this area, which is painful and frightening for the client.