The easiest way to tell is to look at the guides, on a spinning reel the first guide is much larger than on a casting rod. When the line comes off a spinning reel it is in a "Looping Spiral", so the larger guide is needed to "Shrink" the spiraling line as it travels up the blank. The next guide is smaller and so on. A casting reel does not need the larger first few guides, the line is coming off the reel in a much more straight line. Something to keep in mind is if you fish with mostly mono, you want the larger first guide (more common on older Surf Rods) because the mono comes off the reel in a much bigger "loop" ( it does not make the corner around the lip of the spool very well, because it is stiffer). Spectra (Braid) makes the "Corner" much better and comes off the spool in a much smaller "Loop", so many newer spinning rods come with a smaller 1st guide than traditionally. The rod company's are happy to make the spinning rods with the smaller 1st guide ... It saves them money. There are many more differences between a spinning rod and casting rod but at a glance that is the easiest.
Hope that helps,
Allan
