Also a former abuse victim, studying towards a masters in social work...
Spanking can have it's place when a child is young, but only when it's controlled. If a parent feels angry in any way shape or form when they hit their child, then they are doing it for the wrong reasons. Spanking should be strictly negative reinforcement for an action that is considered wrong. It works by helping a child associate pain and consquences of negative actions. Almost every study shows, however, that spanking loses it effectivess before a child reaches their pre-teens.
Having said that, I myself have never had to hit a child. This includes helping my father run a daycare for ten years, and teaching sunday school classes. In my experience, which has also extended to family advocacy cases I've dealt with in the military, spanking is almost always an instance where the parent needs more instruction/technique on child rearing. A platoon leader sat down and explained to me once during a particulary hard situation involving three abused children, "No one is taught how to raise children...everyone just assumes we should naturally know how, and the fact is that very few of us really know how."
Now of course, the natural and first response from any abusive parent is, "well, it's a lot different when you're raising your own children." I, however, have yet to find a child who has not benefitted from being removed from an abusive child raising environment, to more positive surroundings. The sad fact is that in almost every instance, spanking and the rebellious attitude it normally provokes in children, is usually a result of the parent's negative experiences more than the childs.
But, the main point is that a good parent needs to be able to find a distinction between spanking as a deterrent, and spanking out of anger and emotional control. Signs to look for:
1) Calling your child names while spanking: This is actually referred to as emotional abuse, and has been proven to cause more damage to a child then the physical kind.
2) Above and beyond what was originally intended: When the sheer emotion of spanking causes you to swat a few more times than originally intended.
3) Losing control of the situation: A common occurence in older children, who begin to develope defenses against the pain of abuse, can sometimes laugh at or further provoke the parent whie being spanked, causing rage and perhaps leading to further abuse. Many of the worst abuse cases involved children who tried to "catch" the spanking instrument and became victims of rage induced violence.
4) Abrupt changes in instrumentation: If you happen to catch yourself either switching from hand to paddle, or switch, or vice versa in the middle of spanking, this could be an indication of an emotionally angry reaction to the situation.
I myself have arrived at a point where I won't stop a person in public who swats their child on the year. Anything above and beyond that though, and I will make a scene...possibly even beat the person myself. And I'll definetely take the time to chop down a parent who belittles/makes fun of/calls names to their child, because that is simply not good parenting...it's ineffective, contributes nothing to a child's well being, and frankly, not fair to a five year old who doesn't have the wit to defend themselves.