This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
You can use SpeedGS. This is a STARTUP program selector for ProDOS-8 on the Apple IIgs which features display of Boot Volume, Date/Time, and current Speed. Speed is shown as a rounded multiple of 'Normal'-- i.e. 1MHz. For download sites, see Csa21MAIN4.txt .