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Correct. Anything which spins at the same speed as the crank shaft, and is accessible, will do. The operation behind the optical tach is very simple: a laser hits the reflector on the rotating assembly, and bounces back to a photodetector. Each time the photodetector sees a reflected laser...

The crankshaft/harmonic balancer - any other point may introduce ratios and possible belt slip.

For a bit more accurate output: Measuring the crank pulley outer diameter, and dividing it by the alternator pulley outer diameter will give you your pulley ratio (use a metric tape/ruler, easier to do the math [thumbzup]). The tack output of an alternator is pulled form one of the stator...