I always hold the limb up to the light or in front of a window where I can see the thickness in silhouette. I can always tell by looking where the thick spots are and whether I have a consistent thickness taper. I can tell where it's going to be stiff by looking at the thickness.
That combined with reading the grain on the belly side. If you have a thick spot outboard of thinner area it will show up as an island of grain. That even counts through roller coasters and twists. If you follow the back, thickness wise, the ring lines on the sides of the belly will be straighter (more parallel to each other) if thickness is consistent. If you see the ring lines wave in and out, you've got thick and thin spots. That also counts on side to side thickness. If the points of the ring lines are all off to one side, the limb is thicker on one side (in general there are exceptions to that). In general, to have a consistent thickness taper (and side to side width) all the growth rings have to come to a point consecutively and centered on the belly all the way towards the tip. Kind of like chasing rings on the belly side only you want to cut through more and more rings as you get toward the tip. Of course that's not to say you couldn't intentionally make a limb thicker on one side to make it bend a certain way but that would also involve reading the grain and using what it's telling you. I'm finding if I get to floor tiller first while perfecting my taper properly I'm 97% there before I even put a string on it. Once I have it braced and see how it looks braced, I can refine it further by looking at my reflection in the shop window as I pull and exercise it by hand so I can feel the pull. I'm finding I use my tree to weigh the pull mostly these days.
Having said all that, the grain trick only works on a limb or stave. If it's a quartersawn board, the grain is going to look the same all the way down the limb so eyeing the thickness (and using the "feel the taper" method) is the way to go.
