I think water temp and time of the year are key in determining pre spawn. Here in CT, The fish have really kicked the spawn into high gear around May 10. Every year, regardless of water temps this occurs. You need to realise also though that not all fish spawn at the same time. We've typically got fish spawning from late April till late June. I would imagine that down south you have the same thing, a spawn over a 2 to 3 month time period.
It has been my experience that typically the smaller males will be the first to move up to the shallows in big numbers. The fat females aren't to far away though. These fish instinctivley know they must eat and build their energy reserves for their coming task of preparing beds, courting a female, the spawn, then gaurding the nest for around 2 weeks. Generally, anytime prior to actually enticing a female to come to his bed, he's in feed mode. After he finds that female though, he's got nothing but love on his mind, and will usually only stike a bait out of defense.
Up north here we get ice, but within a week or two of the ice leaving the water warms to the low 40's. At this time you can find nice fish coming up shallow to sunbathe and eat something if it comes wandering by. They are still not in real hunting mode. But, as the water goes from 45 to 55 and with each degree of increase in the water temp., their activity level raises. I would guess to say that if your water had reached a low of 50 during the winter and is now 55 and trying to rise, you could bet that there are more and more fish moving shallow to eat and warm up.
To answer your question more precisley, I would say that pre spawn could be considered water temps from 55 to 65. Though, as I said earlier, I think time of year is important. In your area if the fish really get going with the spawn in mid March, I'd say you're into pre spawn right now. Listen to advice, pay attention to the little things around you on the water, take current weather into account, and I'm sure you'll do fine. Good luck.
Feel free to ask for more info.