cool,
how much reflex-how wide of a stave-how long, etc.? My first thought is that if you can't tell if it is centered then it can be fixed later--start making a bow out of it. Otherwise, what I do with a full, rough stave is to find the center on the back, at the tips, and clamp a string to each tip so that the string is tight. Lay the stave on the floor, belly down, and stand over the thing so that your eye is centered, moving back and forth slightly, so that you see the string splitting the tips down the center, looking back and forth at each tip, occasionally glancing at the string going through the handle. So the string is a bridge from tip to tip and you are looking down to see where the string lays relative to the handle.
That same string, clamped to the center of the tip, can determine where the curve or lean is. Leave one tip/string clamped and move the other end of the string to the center of the handle and again observe from above to see if the string stays in the center of each limb, all the way down, or if one or both limbs are curving or if the handle is bent.