Scott:
The best and most practical way is to "put it through its paces": set it up to cool a measured and metered water flowrate, while measuring the electrical draw, coolant utilization, and all pertinent temperatures and pressures.
With the above data, I make a material and energy balance in an Excel Spreadsheet and find the refrigeration effect and the energy efficiency of the unit. This should tell you everything except for the durability or how long it will operate under these conditions. That, I'm afraid, is something you can only find out with a good case history of the unit or with a super inspection job prior to purchase. Otherwise, like all "used equipment surprises", you'll find out later - when you really need the unit's performance.
I recommend you read thread135-127098.
