Usually there is plate or a stamp that the manufacturer put in place in the
transmission that says the transmissions gear ratio. It may be a code or it may say it right there. Your way will
work, unless its a hydraulic
transmission (it slips). If its a wet or dry
clutch turning it manually will work although turning the
engine is the hard way of going about it. If turning it manually is your only alternative, I would turn it with a large socket wrench and not try to turn it with the starter
motor. This will give you much more control. Turn it in the direction the
engine normally turns.