Critical temp varies depending on the steel alloy, somewhere between 1340-2500F (to my eye, it's usually a low-orange color). I don't have a way to measure such high temps in my shop, so I look for a phenomenon called "decalescence", which indicates critical, and looks like a shadow passing through the hot steel. (if you google 'decalescence', there is good youtube video showing the effect.)
As long as you tempered your prod ends after quenching, you should be fairly safe from a prod explosion...good idea to put a strap on too, just in case.
To be honest I found making steel prods a little scary, and after a few, I moved on to wood and sinew prods, which would be a little safer if they failed. They also seem to shoot a bit faster.