How to Calculate Efficiency in a Tesla Turbine?

Yes, the work efficiency measurement will just be work output / work input but you must remember in setting up your measurement system that your measurement must be the appropriate unit, Joule (SI). I suspect that it will be easier to measure the power efficiency but since Δt will be identical on both sides of the equation, the power and work efficiency will be identical. I don't expect the efficiency to be consistent for every load condition. I would expect that you will end up with a multi-variant efficiency curve that changes with fluid flow rate, fluid viscosity, and mechanical load.

The freewheeling (maximum) RPM alone will not be a very useful value in calculating the work efficiency. The only work being done is fighting the rotational friction of your turbine's bearings. Now measuring the amount of time that it takes to get up to this RPM from a stopped condition can tell you the work efficiency of just the turbine. (Actually you'll find it easier to measure the time to get to a large fraction of the peak RPM.) You will have to know or calculate the moment of inertia for the turbine to apply this approach. I do not think that this is a practical approach to measuring the efficiency, but it is an informative approach because it does directly measure just the turbine's efficiency. More than likely you will have to apply some calibrated mechanical load to the shaft of the turbine. Often this is the added energy transfer device of a generator with fixed resistive loads attached to permit identifying the power converted to the electric load.