Typically for an air-to-air charge cooler, we talk about effectiveness, which is not exactly efficiency but it's similar.
For a cross-flow heat exchanger, which is what an air-to-air intercooler typically is, effectiveness is a ratio of the difference between the inlet and outlet temp of the hot fluid (charge air or "boost" in this case), and the difference between the inlet temp of the hot fluid and inlet temp of the cold fluid (ambient air in this case).
So, you need to measure the following temperatures to get a number for effectiveness:
Temp of the ambient air flowing across the cooler
Temp of the boosted air coming into the cooler
Temp of the boosted air coming out of the cooler
You also need to know the specific heat capacity rates (C) of both fluids (both are air in this case).
Another thing that's interesting is the pressure of the boosted air coming in and coming out, and how much that pressure drops as the boosted air flows through all the little internal channels in the cooler. There is a temperature drop across the cooler, which is the goal, but there's also a pressure drop, which is an unwanted side-effect because it makes the turbo's compressor work that much harder to get to the same end pressure in the intake manifold.
A kind of ballpark standard figure to use for an aftermarket air/air cooler is 70% effectiveness. For an average air/water cooler, I use 85% - they are more effective due to the higher heat capacity of the cooling fluid (water). General guidelines for selecting a cooler are that you want to minimize the pressure drop (aka boost loss) across the cooler, and the simple way to do that is to make the cooler as big as possible given the space you have in the vehicle. Another important factor is design of the end tanks and charge plumbing, which both need to be carefully shaped to minimize pressure loss and ensure even distribution of airflow inside the intercooler.