The RPM never changes. Older units run at a constant 3600 RPM for 60 HZ. A lot of the newer Westerbeke's run at 1800.
I know of two ways.
Most digital volt meters has a hertz setting. You should see 60 hertz.
A cheap digital tach works good too. You stick a piece of reflective tape on the side of the crankshaft pulley and aim the digital at it to get a RPM reading. Should be 3600.
http://www.ebay.com/itm/Digital-Lase...-/331075626354
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