I like to look at the top of the splines. The wider the top is, the less wear there is on the spline. It's an easy photo to take, wipe off the lube, and get a light reflection off the top. One spline is all you need to judge the wear. The inner splines are a bit trickier, but the same idea. I take photos now every time I lube them.
Note the nice wide flat surface on the top of both the inner and outer splines here. The inner show a little wear as indicated by the slight step in the side of the spline. At 42,000 miles, there is a ton of life left in these splines.