Most alternators provide a signal that can be used to indicate how fast the alternator is turning.
RPM is not without problems. The purpose of this application note is to clarify these issues. The alternator typically rotates faster than the engine because the engine has a larger pulley than the alternator. A rough idea of the ratio can be obtained by measuring the outside diameters of the two pulleys. However, the depth that the drive belt seats into each pulley must also be considered.